Hay, I can't offer any help, but since my system is nearly identical to yours i'm somewhat worried. I'm running the VIA4.35 drivers and the latest NVIDIA ones. I don't have any problems at all. Just on a guess here, have you updated the BIOS on that video card? Also, have you downloaded the most recent AMD AGP Minport drivers for your board (NOT OFF THE ASUS SITE)
I don't even know if they make them for XP though...Chipset Drivers for AMD based boards
Hopefully something here will help you....
This is off topic, but do you have instability with the Irongate AGP 2x registry entry on your computer?