Compatibality Experience : Once I bought a ATI mobo Rs480i chipset based for my old 478 pin based P4 proc. On that board my nvidia 5200gfx agp failed to work
though no probs with intel chip based mobo.
Ati mobos possibly doesn't supports nvidia cards......
As for nvidia mobos they do support ATI cards but not all Ati cards..
Check your mobo manufactures website to confirm which ati cards are supported..
As for performance Nvidia 8800 Ultra is the fastest one.....according to Digit Oct. Gfx cards Test.