Apple has written, that no 3rdparty g3 or g4 upgrades can install X, but i have seen i various places where it's written X-compatible about upgrade cards.
i have a 7600 upgraded with a sonnet 306 Mhz g3 card and 96mb RAm... will I be able to install X??
On the PB release, it was entirely possible for this to work.
On new releases, not only is it broken, but there is error-checking that seems to imply that Apple has taken steps to (for the time being) prevent X from booting on older systems. (according to macosrumors.com -- not entirely reliable) But then again, the very latest build won't even boot on a beige G3 (again according to MOSR), so who knows about the final.
Finally (according to MOSR) the 1.1 update due in june should include OS X support for many older PCI powermacs.
But this is all thirdhand conjecture. The truth is I don't think anyone knows.