Sowwy Catch :'( :'( :'( :'(
I've just been thinking about your CPU thing, and an AMD XP 2000 is rated at 1.67GHz, I can't find the actual figures, but I'm sure this has a multiplier of 12.5 that means with a FSB of 133, it would be running at 1.67, but with a FSB of 100Hz it will only be running at 1.25GHz, which is a significant loss of performance.
Now if you are using RAM rated at 266Hz DDR (which I'm assuming you are) then it appears your mobo is wrongly configured.
In your BIOS if you can find the PCI and AGM divider settings, do you have the option for setting the PCI to 1/4 and the AGP to 1/2 ? if this is the case you can run 133 FSB as ..... 133/4=33Hz (PCI speed required) and 133/2=66Hz (AGP speed required) The RAM speed should be FSBx2 for 266Hz DDR RAM
Now this will probably be double dutch again, but it is the basics of how the various parts of the system interact.
I'll also have to look for your mobo details, as there may be a jumper on it that allows the FSB to be taken beyond 100Hz. I know this is the case for some mobos, if there is a jumper (check your manual) try moving it to the 133Hz setting and see what happens under the AUTO settings.