Hi Mark,unless it has the slots already in for both types then you would have to use the one thats already in.
Have you got a program called Si Soft Sandra or similar?
That will tell you the make of motherboard and also what kind of ram you have installed and also what alternatives you have available.
Depending on your operating system,system information may give you this information too,XP does I am not sure about the others.
Can you give me as much information as you have that you know of about your PC and operating system please?
Do you know how to get into BIOS,if so that will tell you what the FSB speed is so we can determine whether you are on 100 or 133 mhz