Will this gpu fit and work inside my Dell Optiplex 790 SFF (with its normal 240 watt power supply) ?

Only if it comes with the low-profile bracket. The one shown in the picture is the std. height bracket. Reading the details, it appears that is all it comes with. Check with the seller or find one that has the L-P bracket.

What's your budget? Can you afford a GT 1030? It is considerably faster than the older GT 730. Might even find one used for less.
Dec 13, 2018
I already bought it, people can't seem to reply fast enough and its frustrating, i either bought it last night or waited a couple days to buy it, are you sure, i googled that image and it looks almost exactly the same as a normal low profile, this is so frustrating.

The date on your post says Feb 2. I answered on Fed 2. Can't reply much better than that.

As to the card, the one in the link has a std height bracket. Not going to fit a SFF case. I've had a couple of those Dell Optiplex SFF Pcs. One was a 790, in fact.
Do you have a monitor that accepts display port? If not, you would need an adapter anyway. That card doesn't have the usual HDMI, DVI, or VGA output. Just two DP outputs.
If you don't have DP, you can try using the card by removing the mtg. bracket. Just leave it set in the PCIe x16 slot loose.