Since there is no overclock, make it simple

CPU wattage+GPU boards wattage+10Wxnumber of HDDs. Ignore ssd.

Multiply the above sum by 1.5, giving some safe overhead. This supposes Dell branded PSU graded by enterprise standards. For consumer PSUs multiply by 1.3.

RTX 3080 320W, RTX 3090 350W in average. Bursts possible, so the above overhead is needed.

P.S. I mean, for consumer multiply again x1.3, i.e. almost double safe overhead. You would not need more

The card designed for your system is the Quadro RTX 4000 Turing based card. If you have 365w - 460W psu you do not need to update.

https://www.youtube.com/watch?v=IPYFz8zcG9w

Dell may select PSU at the time of the order, and it may be weaker than the maximal GPU requirements possible on with the platform. Bu t I agree, in most cases with single nonoverclocked desktop CPU and 1 GPU, any current model, 500W is enough.

did you figure out what would fit both in terms of size inside the cassis and power supply?   Looking to see if RTX-4070 or 4080 would fit. 

I’m in the same boat. I have an Optiplex Precision 3630 tower and would like to add a GeForce RTX video card.  Do I need a slim graphics card or will a full-size single-slot card fit in the tower case?

A single-slot graphics card that isn't LP should fit.

I attached the spec sheet.  It has a list of compatible cards and should give you ideas about what sizes fit.

Thanks.  I’m looking more towards a GeForce RTX 4070/ti/super.  Can I assume if their measurements are close to the Quadro cards thaat the 4070 would fit?

Like many GPU's, those 4070's come in different sizes.  Chances are a 2-fan card might fit and a 3-fan card won't.  Check the specs of the card you want to buy against measuring inside your 3630.

I'd rather be more concise than that, but couldn't find internal PCIe measurements in Dell doctrine, just the outside of the case.