Ohhh...I had this problem too. In fact, we have much of the same hardware.
How I fixed it was by reformatting everything and doing a clean install. Yes, that might not be very comforting, but I have a feeling that you have device driver conflictions. I had 4 of them, but Windows always said that there were no problems.
Also, go into your BIOS. Check to make sure that "Fast Write" is disabled, make sure that your AGP Aperture Size is equal to half of your RAM (i.e., if you have 256 MB RAM, your aperture size should be 128 MB), and you should be good at 4x for the speed. In fact, I had troubles running at 2x (I have the same exact ATI Radeon card). Also, I would suggest turning off "Plug and Play OS," because you likely have IRQ conflictions. If you can underclock your CPU (A7A266 boards will let you), I would try that as well. Also try flashing your BIOS to the latest version (currently 10/2001). I had to do a lot to my BIOS to get everything working correctly. 😛
Then, I would slowly reinstall everything. After the fresh reinstallation, go to Windows Update immediately. A lot of the third party drivers from ASUS directly suck. Make regular System Restore points, make sure you have downloaded the latest ATI Radeon driver (check my topic on it in "Hardware"), and make a lot of tests. If you find that the problem has started again, uninstall the driver and make a System Restore afterwards. Uninstalling, unfortunately, leaves a mess, which is exactly what you are seeing now. System Restore points eliminate that mess.
Melon