the 7750 will be your best choice if you dont want to buy a better cpu(recommended) and it's compatible with almost any psu.
You get great performance @720p with that card, you better oc the cpu if possible.
Yeah 7750 or a 7770
Personally i'd go for the HD7770, that way if u decide to up the resolution it's will still have plenty of power for those games
and really you can't go wrong with the 7770 as it can pretty much play anything around (with lower detail on some games)
I have a Phenom2 X2 550 BE (3.1ghz) box with a HD7770, handles everything i can throw at it pretty much