Ok so what sort of budget do you have ? Also what monitor size/resolution do you use ?
Personally i would be looking at a 5770 or a 5750 for a system like that, they are both the latest tech with DX11. the 5750 for anything up to 1440 x 900 and the 5770 if you are looking at resolutions above that. Once you go past 1680 x 1050 Its time to look at something larger like a 5850 but i dont think your PSU would be up to that step.
Both cards will handle a step up in resolution depending on game and settings, the guidelines i have suggested are very rough and give a good margin for extra performance.
Take a look at this review http://www.techpowerup.com/reviews/HIS/HD_5770/10.html