It has the 625 watt power supply and I'm looking for something nVidia of about the power of a GTX 980 or better. My concern is the size of the video card that might fit.
If this is the tower workstation you should have no issue finding something that will fit. I didn't see an SFF or DT listed for this model.
Easiest thing would be to open the panel and see if you have two slots available on the upper PCI-E slot, see what manner of power connector you have, and see if anything else is in the way inside. Measure with a tape to see maximum length you can use.
The length is the easy one. On a previous Dell tower case I had problems with the height of an ASUS GTX1050 TI and the height is more difficult to measure and I haven't been able to find the max height is the specs.
I looked at a spec sheet and noted that all the video cards spec'ed to the machine are single slot.
Going to be hard to say.
In my own experience with a Dell XPS, it had the mobo locked to three specific cards, however I have upgraded other machines in their lineup without issue.
The ASUS GTX 1050 TI fits, card dimensions are 8.00" x 4.50". There seems to be plenty of room length wise and I suspect one of the cards that is less than 4.6 inches tall would certainly fit.
The height is 4.8 inches and the height is the concern with the Dell workstation not the length. There are three fan Gigabyte designs that are under 4.6inches, RTX 2060 1660 etc. I think one should do fine.