MountainDewMan

The 790 SFF has a 100% standard 24 pin PSU connector on the board.  The only thing to really keep in mind is that if you take it out of the original case, then you will always have to press F1 at startup/reboot.  This is because of a security feature to stop case tampering, since these were meant for offices.  But I have heard of some putting tape over that sensor, which will apparently remove the need to use F1.
 
The oddball Dell PSU connector didn't come until the 4th gen, and the 790 is a 2nd gen.  You're good to go.

PSU: in general yes, but not in the stock case (unless it's a direct replacement, obviously). The X90 and X010 were the last generations before Dell went to a proprietary design, because God forbid someone upgrade their PC instead of buying a new one, right?
 
Case: this one's trickier. If you have a custom enclosure, yes, very doable, you just have to make sure that the front panel connectors on your custom enclosure match up with the connections on the motherboard. EZ mode for this would be to simply take the front panel electronics out of your current 790 SFF and attach them to your new case. Failing that, the pinout for the 790 SFF shouldn't be hard to find. I think the front panel are the only non-standard connectors on the 790 SFF board, but you might get weird alarms if you try to boot it with one or more previously empty connectors (so, FP audio, USB, etc.) left open. Nothing that will stop you from using it, just annoying.

from the looks of things, you've gotten lucky on it being standard PSU connectors... as for the "dell may switch up wires".. dell loves their custom connectors, but i've never heard them switching around pinouts on standard connectors.