As Walter mentioned. both cards are equivalent in terms of performance but not functionality.
Choose the nVidia if you want to drive 2 30″ Cinema Displays or want to go up to 4096×4096 pixel projects. Downside is that it takes up two slots so it limits PCI card expansion, and it is more expensive than the ATI.
Go with the ATI x800 if you need PCI slots or want to drive an older Cinema Display (that uses the older rounded ADC connector, not the newer DVI connector). The ATI is also a little less expensive.
Jim Kanter,
Digital Film Institute
http://www.dfilminst.com