PC100 is what's "recommended" but many have used PC133 with no problems - it automatically "downshifts" to run at the slower 100MHz bus speed.Be aware that I have frequently had issues with 133MHz RAM (mostly, the box doesn't recognize it) and almost never had a problem with PC100!
do you know if you were using the 8 chip or 9 chip variety? CL2, CL3, ECC, etc.?
That could make more of a difference than 100 vs 133 - in *general* terms, 133 *should* work fine when 100 is expected, not the other way around, but CL2, CL3, ECC and others often cannot be mixed/matched.
Jale