I agree, a 6600
GT 256mb will handle it well, if you want to go a bit better try the 7600GT, or if you want to be sorted with most games for a while, try the 7900GT, I use a 7900GT and it's so great I could cry lol.
Also as someone mentioned before, I'd recommend 1gb RAM +++, my old rig had 1gb RAM and still lagged a little, just got 2gb and it's a dream, anything below 1gb and you'll think about turning things down.
Hope this helps, and good luck with your upgrade, if you ever need anymore non-WoW tech help check out
http://www.tech-forums.net, I usually hang around there, and the guys there are really good at helping :)