Depending on your budget and resoultion I would suggest going with a 6870,GTX560ti or a 6950 1G/2G.Of course with any of those GPU's your going to have to upgrade your power supply like everyone has already stated.
I would also suggest O.C.ing your CPU to get the best performance if you decide to get one of these higher end cards.
I have gotten my 965 to 3.8ghz and i'm sure your chip can be pushed to 4ghz without to much effort as well.
ionut19 "BS" describes the the condition of grass after having been processed by a bull's digestive system. But I think swifty is saying it because you said monitor size doesn't matter.
Resolution size is what matters although usually larger monitors have higher resolutions.
Chemericash you need to edit your post or submit a new one and let these guys know the specifics. (including your specific monitor resolution, budget and games you play). No sense asking for advice and not giving them enough information to give it!