Have you downloaded and tried AGP Wiz which is a special Creative Labs only product for their video cards. It permits you to check AGP 1x, 2x or 4x. It also allows you to enable sidebanding or disable it. I think you should check it out.
Also, just for laughs, go to BIOS and set the AGP Aperture (if you can) for a very low value. ie: 4 megs.
I'm curious what you find out with any or all of these things. I believe the Athlon is your key.
BTW, I use a CL GeForce 256 (32megs) DDR without any crashing problem at all, however, I'm using an Intel 440BX-2V motherboard and an Intel 600Mhz with main memory of 256 megs. I overclock the GeForce to its maximum and on a recommendation of HiTech I turned OFF all mipmapping.
Regards,
Badger