I agree with BadMange. I use an MSI GeForce 4 TI4200 softquadro'ed up to a Quadro 4 700 XGL. Works great with Maya.
Not sure how noticable a difference there is between the stock card really because it worked fine before I patched the nVidia drivers. Less problems with paintFX, maybe. Gaming doesn't suffer that much...I haven't noticed anything on the occasion I fire up Quake III.

Regarding ATI, not sure about the newest card but in the past they've had lots of problems within maya. Just go through some of the hardware threads at highend and you'll see what I mean.