Honestly, you aren't going to see major performance increase, but if you're worried about temps or would be more comfortable running lower it can't hurt. I must say I'd rather save up for a new one than spend money on a cooler.
Arctic Cooling Accelero S1 Rev. 2 with the turbo modules would be a cheapish choice. That's what I would get if I didn't have this Asus Glacier fan that seems to keep mine quite cool while oced.
Honestly, you aren't going to see major performance increase, but if you're worried about temps or would be more comfortable running lower it can't hurt. I must say I'd rather save up for a new one than spend money on a cooler.
As previously mentioned the Arctic Cooling Accelero is good, or you can get their Accelero Twin Turbo. GPU reports 20C cooler than before when under load, and that's with the fans running in silent mode! So well worth it.