Oh, why don't JUST the processors have the multiplier, instead of having it on the mobo? Well, your memory, north bridge, south bridge, and everything inbetween all have to talk at the same frequency... The core speed (crystal), along with the multipliers and other things sets the speed of which EVERYTHING works together. That is why you have to watch your AGP bus while overclocking, because you can set it too fast, and the video card just can't keep up.
Tas.