PCI-X is just an overclocked PCI card, and those machines still had AGP video card slots.
If it is a PCI-Express machine, I thought those video cards were a little newer and were the Nvidia 6600, 7800, & Quadro cards ... with the ATI 9600/9650/9800/etc. the last of the AGP cards from Apple.
http://themacelite.wikidot.com/compatibility <- AGP OEM/Flash etc. card matrix