Actually I meant change in BIOS.
BTW what kernel are you running, I would suggest trying the latest vanilla kernel.
I don't think I can change it in the BIOS. All I can change pertaining to the AGP setup is aperture (32-512 MB), SBA (auto/disable), and FW (auto/disable).
But I did use the trick from the NVidia readme to compile the driver so it won't use anything higher than 4x. It didn't help.
I am running a custom kernel compiled from the 2.6.8 sources with Debian patches.
I will attempt to install a vanilla kernel tonight. I've had trouble with that on Debian in the past, since they use a cramfs initrd and there needs to be a patch applied to the kernel to make it work, but if I am careful about including drivers in the kernel so it can mount the disks without modules, I can install a kernel without an initrd.