Thank you for the reply!

The reason I believe this should be fine - and I would guess one of the reasons this other user recommended it - is that while the gpu's spec recommend a 300W supply, it only actually consumes 30W - and the "blue" PCIe slot on the 790 is spec'd to handle up to 35W., per the Technical Guidebook:

https://i.dell.com/sites/doccontent/shared-content/data-sheets/en/Documents/optiplex-790-tech-guide.pdf

on page 19.

The 240 w psu is too small to support any of today's high powered video cards even the 1030. Most cards require a minimum of 300w including the 1030. Remember that the psu has to power everything on the computer, not just the video. See the specs-- https://www.geforce.com/hardware/desktop-gpus/geforce-gt-1030/specifications  

Use the integrated video or the added card that came with the computer. The 790 is old now and you should be replacing it soon. 

Thought I'd wrap this thread up with a final report.  I did install the GT 1030 and it works perfectly.  Vastly improves video quality.  Fits like a glove in the SFF case.  A tiny little bit of fan noise - inaudible when anything's playing.  Hasn't overheated or over-drawn the psu after several hours of video watching.  A major improvement.

The official GPU's are in the Tech Guide.

http://i.dell.com/sites/doccontent/shared-content/data-sheets/en/Documents/optiplex-790-tech-guide.pdf

M0KV6

1GB AMD RADEON HD 6450 with DP and DVI Optional card

512MB AMD RADEON HD 6350 with dual DVI or dual VGA Optional card

https://www.dell.com/support/home/us/en/04/Drivers/DriversDetails?driverId=KXVDP
(LFH  adapters convert to dual DVI or dual VGA)

The GT1030 GDDR5 is a much better card than those.

There is a DELL specific version of this card

spec is 25W TO 40w for the Video Slot.

That's why the Radeon HD5450 at 19W is fine.