Here are a few things that might help you:D
hd 7770 ghz ed, most powerful of the cards, system uses about 250w at load
Personally I'd say that the 7750 and 550 ti are tied, but I'll say the 550 ti is the second most powerful as it performs better in a few games such as bf3 and portal 2
so that being said, 550 ti, second best performance, system uses about 290w at load
finally, the 7750, third best, system uses about 220w at load
The loads are the entire system not just the gpu's, hopefully they use the same test bench for all their tests, so from what I see here it that if power is your concern just throw out the 550 ti, and get a 7750 or 7770.
go with the 7750 if you want to save a little money or the 7770 if you want better performance, the card dscudella recommended would be a good choice, its a dual slot card, but if the case and mobo are the same as a picture I saw, it should fit just fine