Actually, memory speed doesnt matter. If you put a higher speed memory on a lower speed memory motherboard, the new memory will operate either at the highest compatible speed suppoted by the motherboard or operate at the speed of the other memory modules. For example, I recently put in a PC 3200 with a PC 2100 (to make 1 gig) and the new memory (by design) falls back to 2100.
What is the STOP error you are seeing when you load xp? To see the STOP error, access the boot menu and select "Disable automatic restart on system failure". Write down the error code that appears and any descriptive information (should appear the line before the code in caps)