If your motherboard supports 133MHz ram, buy it.
In two words:
The difference between 100 and 133 is that 133 has a faster trasnfer speed than 100. So operations on your mac r faster.
If u hav a 100MHz MB, u can use 133MHz ram (that will be used @ 100MHz).
If u hav a 133MHz MB, u hav to use 133MHz ram.