I really know nothing about the ATI Radeon cards, except that they are not on the on Alias Maya 5.0 Hardware Qualification List. As far as NVIDIA go; for maya work, you should probably look at a card no lower than the Quadro4 750XGL which retails for around $350. However, if you have more cash available I would highly recommend looking at the Quadro4 980XGL, QuadroFX 1000, 1100, 2000, or 3000 cards which range from about $650 up to $2,995.

I personally run a Quadro4 980XGL and it is a superb card for Maya as it runs lightning fast and is extremely stable. I've heard nothing but good things about the higher end cards (FX1000+) so, if you want to spend, you cant go wrong there.