Hi,
I'm having the same problem with my Sapphire Radeon 9600XT 256Mb.
I load up a agame, go into the actual playing mode (3D mode) and after a short period 5-30 seconds, it crashes.... the whole system. Requiring a manual reboot.
I've been looking all over the net but most forums have said turn Fast Write off, which I have. Others have said it's a VIA problem. I've got all the latest Drivers, and even checked for any SOYO bios updates I might have missed.
I'm not gonna change my motherboard anytime soon.
Some VIA forum users stated it could be AGP voltage or even a weak PSU.
I don't really wanna mess with the AGP voltage.
When i send the error report to Microsoft after the reboot it comes up with exactly the same error report every time which states something about a thread blocked and it's something to do with the driver.
Tried changing a few BIOS AGP settings but to no avail.
Tried the current Catalyst drivers and then tried the OmegaDrivers and they haven't made a difference (apart from in DXDIAG the tests run Uber fast).
Theres no worry about driver conflict as i had to uninstall the catalyst before installing the Omega driver.
The only difference with my problem is that when i put my GF4 Ti4600 back in, it runs fine as before.
I'm running:
Windows XP SP2
AMD Athlon XP2600+
SOYO KT880 Dragon2 w/ onboard sound
2x 512MB PC3200 Corsair Value Select in dual channel mode
DVD ROM
CDR/RW
C: IDE 12GB
F: IDE 80GB
Asus Wireless LAN
Sapphire Radeon 9600XT
400W PSU
(on a side note, with the GF4 my wireless connection is pretty poor, ~30-50%, but with the radeon it's Excellent ~75-95%. strange.

)