Depends on what your ok with? Do you want an overkill gaming machine or something that can simply max all games at 30+ fps?

Right now I'm on a cheap Athlon 64x2 @2.7ghz, HD6770, 3Gb ram and I play with everything on max settings but with Ambient Occlusion and Depth of Field off, 4x MSAA and low shadows @ ~30fps. I would likely be able to turn that up and still keep above 30fps if my cpu wasn't so horrid (my bench score DOUBLED to over 3k when I switched to a amd Phenom II x4 965 BE). This rig is cheap and definitely not overkill but it pretty much lets me play at max settings. With that said my suggestions are this.

Cheap but max settings.

HD6870 $167.56 shipped

A good bit more but max settings, leaning towards overkill

HD6950 $230 shipped

Anything beyond the HD6950 isn't needed for any game unless your resolutions are beyond 1920x1080 and or multiple monitors. Keep your processor, it should do just find as my bro played 50+ fps on a the 965 BE I mentioned, 4gbs ram and a HD5850 with everything on max, Ambient Occlusion and all that included.