Dell 790 Optiplex PCs  (MT/DT/SFF/USFF) circa 2011          

Uses a Gen2, CPU, I5-2400 is common, here, and can be upgraded,  This CPU runs the RAM directly. CPU 2011 made.
Chipset intel Q65
Comes with a 265watt weak PSU now upgraded to 750w.
DDR3 (1.5v) but can use some modern dual voltage  1.5/1.3v RAM)  The RAM  type is dictated by CPU and BIOS upgrades for PnP to work.) (see DDR3 density rules here)
DDR3 memory can be a challenge to find in year 2020 for sure new (Do not buy old DDL3L 1.3v sticks from the year  2010 era they will not work, buy only new sticks or 1.5v only sticks)
Max DDR is 16GB , Kingston says 32, and I do not believe them. $85 to find out, and not me , not ever. (opinion here)
Do not mix RAM sizes , RANK or density. Use a matching set of DDR RAM or go crazy with a dead PC.(I warned you ! and so did
Avoid all PC cases not MT. MT has full power, PCI-e slots 75watts not lame. and 4 DDR slots not just 2,  I buy and cover only MT cases. (case means form-factor)
The MT allows all GPU card choices not just low-profile 79mm slow cards.

This is only a test bed (jig) PC and not used  to do real PC work of any kind it is for my test lab. 
(I have tested many  GPU cards here (over 30+) up to GTX1050/ GTX1650 Turing class and R9s) and all run perfectly even 250watt cards !) I continue to test more GPU!
Runs W10-64bit all day long, no issues ever. (it installed and Microsoft PnP plug and play and hits the deck running perfectly.)
The video card, needs a drive from the 12 makers of your card if present.
I now run a Corsair TX750 PSU with 62amp 12vdc single rail (new technology is best) $50 PSU are winners, if you run high powered GPU card like RTX series.


Many folks still use this great 790 , it was made in year 2011 and is not too old, it came new with W7 and I upgraded with BIOS v:A18 is UEFI certified
I turned off UEFI fully off in BIOS, (fastboot, secureboot, safeboot,TPM and boot UEFI all disabled.) but... but turned on  CPU advanced features in BIOS for sure NX (no execute enabled)
All tests done with SATA SSD and formatted, GPT mode. (64bit does that automatically) no need to format the drive, just let the windows installer do its THING.
Intel Q65 Express South bridge (chip set and very PRIME) and CORE gen 2, up to 95watt CPU.
CPU's Up to I7-2600 and 3.4GHz. clock. 4cores and 8 threads.   (jump to GPU test results)

Here are some facts about this PC. ( this is my GPU card report)
The PC needs to be BIOS upgraded to A18 version or higher, I use Dell DDDP DOS, to do that and run O790a18.exe via a DDDP boot USB stick. Done. (sadly modern F12 BIOS ,BIOS Upgrade method is missing here)
Some  Dell PCs must be BIOS upgraded in steps or it fails.  A2 to A8 then 8 to 12 and last to A18 . (Dells documents are horrible in this regard and in that detail) sadly.  Flash the BIOS with UEFI turned off and no Bitlocker enabled.
The PC will have dead BIOS screens if he RTC COIN cell is dead, Below 2.9vdc is a bad Cell. (at 0-vdc the power button is dead)
Plugging in the monitor to the wrong video port? mine has 4 ports, only those in the X16 slot are live below(or using lame VGA adapters is prime #1 to fail)
The BIOS hotkeys, F2 and F12 keys seem dead? check the coin cell first. (seeing F1 warnings is not a dead PC and for sure not DEAD BIOS)
If you have BIOS set to fast boot (mines off), it can be hard to see, use HOT KEYS,. use FASTBOOT (in BIOS) set to "Thorough" (if you can reach it) by you hammering the F2 key quickly after power on is pushed.
The F2 key is best hit at DELL 790 logo instant !
To attain BIOS screens more easy unplug the HDD 0 (SATA port 0) now you have more time. (easy no?) (if you see , no boot media errors now,then that means you are too slow hitting F2, so try again.)
Do the long HARD reset if you have problems with BIOS acting dead or power button acting dead ,lamp or blinking oddly. (set BIOS defaults on any RAM/CPU changes too)  The RTC COIN cell battery is no good now, 9 years old +

See all GPU cards tested by here at the end of this short page.

Using my test bed frame , Bit coin sucks frame, I call it.
This lets me test any Mobo easy, and any GPU card I want,  with a 750watt PSU.  (12v single rail 62amps max)
(Learn that the PSU only uses power on demand, just like wall jacks in your home A.C, learn that if you have power lacking the OS will SOD or the PSU SIMPLY SHUTS OFF ON ANY OVERLOAD)

See GTX1050ti run here

The beauty of my test bed jig is all PCI-express. cards made fit the X16 slot and no hitting. I can even plug in 1/2 height cards and not need to remove the J-bracket to do that test.

The full sized MT desktop uses the full sized tall card. ONLY !  No 2 VIDEO cards have the same 3 dimensions,  that includes depth ( even 1 ,2, 3 deep cards are made) Do not guess, but do MEASURE your space with a RULER.
There are over 12 makers of cards and many have 10 versions of just one card !  Do not think this is like changing light bulbs, the card had to fit first and not overload the PSU 2nd.
The most easy PCs to own are full sized full height cards.  (if it can't fit or hits things , the card is useless to you)<< some guys cut out the DVD drive cage and HDD cage to fit larger GPU and or Larger PSU.
See My PCI-express card size page here. (full spec) for sure heights and more...

UNDOCUMENTED SECRETS: (unlike major mobo makers like,  Gigabyte, MSI  that have great documents) Dell (or any OEM) has nothing to say on proprietary pin headers or jacks.

Best and good  is having  a front panel I/O below, have  this so you can view 4 P.O.S.T error codes  (4 LEDs below bottom)
The BIOS will nag you with endless F1 errors. if things are missing. (but can be skipped) 
The mobo may need its own front panel with correct Dell DP/N: model number, the cable end, It is keyed and must fit or you have the wrong panel, (DELL Hates parts list for PC's, so buy HP)

The secret 34pin jack for I/O panel. (This only one example, other PC use a different panel ,totally)

See the DELL 380 PC with its odd ball 40 pin Power panel, (1/2 hacked by me, just to make it work, I did not trace out sounds(mic/lineout))

Some dells have  power switch others only have the one seen above, 380?  in the power panel.  (NO ATX standards here,  they do not exist)
I offer this to you if your power switch is bad or missing and you need to jumper pins 1 to 4 to turn it on.
The SFF 24pin power jack hits many GPU double wide sized , PCI-e GPU  card fan casings it.
 The DT/MT mobo a wide card can hit the 2 SATA ports., but 90degree angle ended, cables may mitigate that issue.

The below card is  DT/MT has old PCI legacy slot (cream color) {not express}  unlike the SFF with none. The USFF has no  PCI slots at all.  Note the 4 PCI SLOTS , 3 are PCI express, the cream one is legacy only.

Mobo bottom here?

The bad case to own is SFF or USFF (small form factor) see GPU slot no room. (and weak PSU) USFF has no PCI-e slots at all. (horror?) Avoid PC's like if you plan to upgrade one.

Using the wrong fan: (Dell does not use Intel  fans and is wired custom to Dells needs)
The Intel fan works ok, wired correctly, but DELL BIOS will not like it  for RPM and sets errors in the BIOS log, but can be used on a pinch. (for sure ePSA fails but we can test with the wrong fan)
The Dell BIOS has table inside to compare RPM to Dells outputted, PWM drive, to pin 4,  if it does not match DELL knows the FAN IS WRONG OR PACKED in lint or bearings are seized. (by DESIGN)

Buy the MT and then most GPU card fits. ok?
his  PC will run with no I/O front panel, .(but will nag you about the missing panels, missing thermal sensors, and missing power LEDs and wrong fans.)
The other nags besides thermal sensor 1, missing is HDD fan missing so I put yellow jumper wire from CPU yellow pin to main chassis yellow pin 1(taco-sig) and bingo ,it work no errors. (Dell secret told)
If it nags, you ? then hit F1 key then F2 or F1 and F12 and use F12 to boot any device you want.
F12 boot on the fly menu, lets me  load W10-64bit no problems.  (f1 then F12 and boot to USBs stick)
I do not USE UEFI modes. (all are turned off in BIOS) to gain my needed full usage as a test bed for hardware.

All tests with 750watt CORSAIR PSU. (To upgrade the PSU is easy,  and cut out the silly PSU lock and trash it) Up to 750w easy ,over that you get extended length PSU and will not fit many PC's. (use a ruler)
ATX spec size is here: (only LENGTH varies !)
This PC runs Windows 10-64bit,  1909+,   )  Dell does not  support full  UEFI on this PC.(proof)
Turn off any found UEFI features and  use legacy mode.
My PC here is fully BIOS upgraded. (and runs legacy with no problems ever)

The GPU cards tested below by me.

Tests done are , VGA and DVI-d both work in BIOS and W10, and runs FurMARK and passmark 3D. (Results all work perfectly)

I tried over 20 GPU cards in x16 slot,  and could not overload the slot, All cards are 75watts and up. (Furmark/Passmark and AAA+Games tried) The MT never failed.
I am pretty sure the 4 slots derate each other, due to the discovered signal power trace found by special testing.
If you use only the X16 , it works just fine.
Missing now on web site is this greater DELL™ OPTIPLEX™ 790 TECHNICAL GUIDEBOOK - V 2.0, page 19
The horror #1 seeing this.
If you read ATX spec, they do allow small form factor PCs to have only 25 watt slots,  but is not any MANDATE EVER.
I COULD NOT GET THE X16 slot to fail, dispite huge efforts on my part to so and have the tools to do all that.
Over 12,000 folks also did not fail with upgraded GPU cards.
The mobo and GPU both have huge capacitors help prevent that , V drop.


Testing 123:
Not only did I compute what it takes to run 75 watt slots (+12v) using LT_SPICE, the  answer is 10mm wide end to end  or more is needed. (+- 10% ATX spec rules)
My personal PSU beats this spec by 10 fold (DPS PSU at 1%) and magically runs this mobo with weak power slots. ( cleaner +12vdc can do that)
I also measured the voltage drop on MOBO naked and benched, I measured the drop and using Van Der Pauw. math computed width average,  I got 4mm. ( that be lame)
We also took a dead mobo and stripped it down layer by layer to see what they did,  the answer is the path is tiny trace looking like me following  drunk sailor.... 
The DELL mobo failed this test both ways, but they warned you above, so we can not complain, if you read the spec, that is...
But if you run only 1 PCI-e card here, I found not problems at all. (huge testing to prove that)

GPU CARD TEST RESULTS:   See my full list of cards tested on all my Dells.

(All testing done only on  MT PC and motherboard.

The stock iGPU VGA port runs 2 FPS (Furmark, slow as a snail) but this is power/functional, report is not  about SPEED ! But first we overload the GPU, (means run it under max load)

The GTX260 is a power hog at 200watt, and gets real hot at full 3D loads.
  This 260 card hits the SATA port cables. (90 degree cables cured this... easy)

If your monitor lacks the need VIDEO port,  use ADAPTERS.

The reason it failed in all my long tests was this adapter just below. (a $1 adapter DVI to VGA , is passive and simple)
Some high end video cards have this huge heat speader on the rear, and will hit the DRAM banks seen here. ( the FIT and not HIT #1 rule just got more complex)

DVI-I to VGA for monitors that have no DVD-I or DVD-D ports. (love to fail, if the PC can not see the DDC/edid data or makes no sense)
Simple cure here.

My parts list: (Dell fears parts lists and I don't know why, HP wins that Quality metric big time ! at HP parts surfer)
Dell also has very limited parts to sell you, even owned for 1 day. (no PSU upgrades, only 1 or 2 GPU cards supported or not sold now) In fact 1000s of  cards GPU work in this PC.
Dell does not do QVL lists for supported DDR3, nor for CPU, they just tell you it is business computer and they don't do upgrades (a lame excuse for sure?)

See adapter horrors here (and lots of cures)

Here is proof, GPT boot drive works on this PC. (just as a test) And the W10 install F12 screen pick to get GPT.  For sure BIOS version A18 up.

This page is for those folks stuffing  any old Dell motherboards found on FleaBay? , into any newer alien cases
Some Dells are BTX not  ATX mounting holes,  and some  USFF case type motherboards have no PCI-e slots !(x16)
Avoid all DELL motherboards with the crosswise (45degree) CPU shock and even more alien crazy queer heat sink fan usage.

Dell buying advice:
The BTX avoid list is here:
Optiplex, XPS and Dimension numbers, 755,780,380,760,3100-e310,3100,5150,9100,9150,9200, xps 200s/210/310/400/410/420/700/720, E520, E310, at the least.
Using a DELL mobo NEAT like that , it complains (POST) bitterly about missing front panel, wrong fans, missing fans and missing thermal sensors and POPS  F1 errors until you fix that.
Run for the hills ! BTX motherboards boards are junk, do not try to use them in modern alien (to Dell) cases. The Heat sink is BTX ALIEN !
Buy the whole PC and be happy, but never buy just the MOBO seen above.  See the crosswise CPU , RUN  away !

DDR3 HELL (1.5v memory only allowed or modern dual voltage DDR) never us DDR that is old and only DDRL3 1.3v only spec'd.
Only use match sets of DDR  1,2,4GB sticks , Dell does not allow 8GB sticks per there manual. ( that  limit  maybe only 2011 limits that year)
The below is intel spec, not Dells, Dell limits RAM by BIOS PnP allowances, so upgrade BIOS first. BIOS scans the DDR SPD chip if it likes what is sees there, I may  work.
This is the Intel, MMC logic  rules deep in side the GEN2, CPU. (hard  pure logic) after this logic the DELL BIOS must like it too.

Kingston shows 8GB sticks that Dell states does not work, (IDK)
2 x4GB is $44 set, 4x 4GB is $88 why spend this on MOBO sold for only $20 now,  why not upgrade your PC, to at least GEN4 Haswell PC status. (new PC or mobo)
Going to 32GB I have no hope there, but IDK.

I have a special PSU inductive ammeter so I can measure AUX power(current in Amp.s) directly.  (To use this with GPU-z and the Shark to double check which cables use what amounts of current) Furmark has all that build in.
See my new W10 install limits page here, (shows even the above MoBo  runs w10 64 bit !)

A better low cost PC is the DELL Optiplex 3020 MT for low used cost of  $50(no HDD). Best bang for the buck and very much upgradeable, and most GPU cards fit inside and even RUN right.

version: 4 .  7-9-2019