280X(7970) would be your best bet if your an overclocker as it's silky smooth. The GTX 770(modified kepler 680) is best if you dont plan on OCing, but just want pre-built power hungry card. And last, the 760(modified kepler), it is almost as fast as a 670, but it is slower. If you want to use any of these cards, then you would definitely need to upgrade that PSU. Ko888 should be to the rescue for a very explained PSU estimate. But personally I'd buy a 550W-600W PSU, and a 770 since I dont think you want to OC.