I've gotten some weird things with my X-800Pro lately when I would reboot sometimes, that I believe are driver related..something a format would have fixed..but I have it in another rig now and the weird artifacting I was getting is gone now.
Things I tried, didn't help... but might help you
Setting AGP to 67MHZ.
New Drivers.
Increasing AGP voltage, and then lowering it.
Using a different molex connector from the power supply (It is plugged in all the way right?)
NOTusing ATI tool..this seemed to help some, cause I had it set too booot at default and then for 550x500
for 3-D apps. Changing clock frequency or resolution would always bring up the glitches..