*Correct me if I'm wrong, I'm really not sure what I'm saying lol*
I think a mobo supports both 100mhz and 133mhz bus speed. Then, according to the processor being used, this bus multiplies. I also think RAM takes a share here, if you're going to use a processor that has 1000mhz FSB , but you have DDR333 RAM, then the overall bus decreases to 800mhz
*Omg that turned out terrible! someone! please correct me lol*