It depends on the resolution and scenario as to where a bottleneck lies. Anyway, I would say that a 5770, a GTX 460, or a 6850 would be the way to go. Anything more is likely to be held back by your CPU in most games. If you're using a 1080p monitor or above then I would say a single 6870, 5870, GTX 470 or GTX 560 may be a better option depending on the games you want to play and how long you plan to keep it, but it would likely be best to overclock your CPU if you want to use one of those.