Sorry....Wrong. Mac Pros support nVIDIA Quadro 5600 and Quadro FX 4800. I wouldn't buy one though, way overpriced for the little benefits you get.
Don't automatically assume you have to buy a ATI 4870 as the better option. Whay not get a 285 GTX from nVIDIA from Newegg for $440?
-J
Poly-pushing on a '96 SGI O2