I have a ATI Radeon 4670, and it bottlenecks. I should've bought something better.
I recommend at least a 5670, or preferably an ATI 5770. The 5770 is Strongly recommended.
Don't get anything more powerful, our CPU is too weak.
Whats the Maximum Resolution of u're monitor?
And Btw, u almost definitely need to upgrade ure PSU, no matter what you get, minimum to 400 W. Preferably, get 450W or 500 W.