133 mhz memory not working together with 100 mhz memory on MSI K7Tturbo?

I have 2 128 MB DIMM's on my motherboard...
Those are 100 Mhz
So I want to add a 256 MB module, but it's 133 mhz!
It seems this means trouble...
Is that normal?
Maybe it's because of the different speed!
Can I change the speed, so all memory operate at 100 mzh?

Tnx
Gert