If you can overclock the CPU it can handle cards up to a level with GTX560Ti or HD7850.
What kind of overclock are you thinking? Increase to around 3.4Ghz? 7800 series would be great, I was thinking more in the 6800s. How much boost would I get from an increase in RAM as well?
I like hanging a gen or two behind the curve as the components are cheaper. Buying games a year or two after they come out helps this as I never need cutting edge parts (and again, keeps costs down).