If your budget permits, a Radeon HD4870 or nVidia 260 / 260+ should be good.
Of course, it also depends on the games you play (not just how frequently you play them). If they aren't too demanding (e.g. WoW), a 4830 would be more than sufficient. If you play Crysis, well then the 4870 would still be stressed. If that's your game, and you haven't actually bought the 550W PSU, you might consider a 650W-750W PSU so you can run a 4870X2.