Yes, those details Andreas mentioned are worth noting.
There are PCI-X video cards, but I've never seen one working in a PowerMac (in fact most of the applications I've seen were in Sun server hardware, not PCs).
And no one seems to have sorted out use of PCI video cards in G5s (they just don't seem to work correctly), although PCI video cards do work in older MorphOS system (including the G4).

I've actually considered flashing a Radeon 9200 with an Apple rom to try it in a G5 for W3D support, but then I like to experiment.

BTW - While the X800XT is the fastest card currently supported, the 9800XT is no slouch.
And, depending on whether not not AGP variants are eventually supported, you might be able to install a Radeon HD 3850, 4650, or 4670 in your G5 one day.