Willie2:
the  chipset on your motherboard allows for setting memory speed independent of the FSB.  In other words you are running your Athlon at 7.5X100MHz FSB but you can run PC133 memory at 133MHz separate or independent from your FSB setting.  If your memory is PC100 then set it that way in the bios like Tetsuo316 said.  Your Athlon cpu probably would flake out at much beyond a 105MHz FSB regardless of which memory type used.
Super6