I recently had a system running stable for a while with the memory running at 100Mhz. I wasn't trying to do that, my bios was being a bit tricky. Some of the Athlon boards are particular in the type and speed of memory used. If you can help it, use PC133 CAS 2. The 100Mhz ran fine on my system, but it may not work on yours.
My System Specs at the time:
FIC AZ11 (yuck I dispise this board)
128MB PC133 CAS3 running at 100Mhz (I've since got it up to 133)
15.0GB Western Digital ATA66 7200rpm
Windows 2000 SP1