The fastest video card you can reasonably afford is "best". You may not get more framerate beyond a certain point, but you may be able to make the images look better with higher res textures, higher screen resolution, and antialiasing.
Anything over an nvidia geforce 6800GT should give you really good results. Any of the nvidia cards selling for over about $140 (8600/8800/9600/9800) would be fast and should let you crank up the image quality quite a bit.
Of course, the rest of your system also matters. AH framerates are at least partially dependent on cpu speed even if you have a fast video card, and having at least 1gb of ram is a really good idea. 2gb of ram is pretty cheap nowadays.