HELP! Turning on AGP causes hardlock/vertical lines across screen

If anyone could help out with this matter I would be much obliged. Here are my specs.

asus p4v8x-x
2.8 533fsb p4
1280 megs of pc2100
ati radeon x800 pro 256mb

I'm running everything stock with all updated drivers (chipset, tried with newest cat's and omega's, newest bios revision). After a little bit, I found out specifically that it is turning on the AGP slot in the Smartgart tab of properties that causes this problem. I'll test it out on the FarCry flyby demo and anywhere from 1 to 20 seconds into it, vertical lines drape the screen and I can't alt+tab out. Hope someone can help me out. Thanks in advance.

One point of note; I don't know how to up the AGP voltage on this setup and I've heard that VIA chipsets have repeated problems with AGP voltage setups.

Have you tried going back one version on the ATI drivers? Completely remove the existing ones first.