It really depends on how much money you want to spend. "Tom" has an article "Best Graphics Cards For The Money: September '09", and a chart that compares "Graphics Card Hierarchy Chart".
He says to go up at least 3 levels so you would be look at "9800 GX2, GTX 260, GTX 275" or "HD 4870, HD 4890" at least. Might as well got for a ATI 5850 or 5870. It will keep you gaming longer.