Thanks for this quick answer. This means we can only profit from the updated graphics drivers under Bootcamp running Windows XP (or Vista?).
Pretty much. Apple makes the drivers for the Mac platform. I imagine they get help from AMD/Nvidia. Since Apple has no incentive to support off the shelf cards, we only get drivers for cards they plan on (or currently) support.
The latest information I have seen says there is G92 and RV6x0 support in 10.5.2. Of course the only system that benefits from that is the Mac Pro, so we will see.