If I'm watching the right motherboard, I think you can install only gpus that use a single slot, and probably low height .

I think there should be some gtx 1650 and 1660, the usual quadro ( p series up to p4000, by example. rtx 3000 and 4000 likely), and the amd equivalent

Also, check your PSU label to see which one you have.  I couldn't tell from the photo.

I was checking the wattage of some of the listed GPU's.  If the PSU is 180w, I wouldn't chance 45w-50w 3D GPU.  Watts, or tdp, of GPU's can be Googled.

Thank you for all the help and feedback so far! These are all things I had no idea about. I took a closer picture of the two labels I could find. Given the model number and output warning I am guessing it is 180W.

Thanks for the photos.  It is 180w.

Side note : if you don't plan to play at fullscreen 4k , but at full hd or windowed, and also don't plan to put all on ultra, the p600/gtx 1030 still do an acceptable job ( I've a p600 and I've no issues gaming )

Given the psu, and the fact that the quadro k1200

https://www.techpowerup.com/gpu-specs/quadro-k1200.c2641

is rated for 45W , it's safe to assume that any card up to 45W is fine without changing the psu.

The quadro P600 / P620 is 40W , by example

The quadro P1000 is 47W , so probably would still be safe

The nvidia gtx 1030 is 30w ( there may be different variants of the 1030 )

Again, there are ati equivalent cards. For any like a 1050 etc , you need to change the psu to something > 300W

Thank you for the great replies! So leaving everything else as-is, on a 180w power supply I'd need a low profile, one pci connector, 45w or less GPU.

If I replace the power supply with a refurbished oem 315w unit (VX372), the GPU still needs to be low profile and single pci slot, but what power consumption could I safely go up to? Or there other, higher watt slim power supplies people use?

I think I'm going to go with this bigger psu direction. It's been shown a 1050-ti will work in this case and with a 315w psu, but hoping to determineif there are any slightly better gpu choices like a 1060 6gb or something.

https://www.userbenchmark.com/System/Dell-Precision-Tower-3420/20842 

The userbenchmark link above might also give you ideas.  Not sure why the link mentions "tower" when the 3420 is SFF only.

With a 315w PSU, you can go up to 75w for a GPU, may be 100w seen as a 275w PSU can still power a 75w card.

If you were to use a 315w PSU from the Optiplex XE2 SFF, both the PSU and your MB do have the 8-pin connection.  I can't guarantee the fit inside the case.  Screw holes will likely not line up.  I don't know if any Precision SFF's have 315w PSU.

A few go might go with a full size PSU, but then it has to sit outside the case.  The same with GPU's that are too large.  They can sit outside the case with an extender cable.

Towers are the best bet for upgrading.  Much easier to upgrade PSU's and GPU's.  And some Precision's already have powerful PSU's.

You're also welcome for the replies!!  : )