You can play around with AGP Aperture settings in the bios as well. I had a hard time with a Geforce in a BX board, I ended up setting the AGP Aperture to 0 MB to force it into pci mode, once I did that I could play Quake3 or any other 3d game without lockups.
But I'd try different values and see what happens, you shouldn't have to go much more than 64 MB.
