You need to get a processor to suit your motherboard. You will need to know what socket the motherboard is and as your friend says you will need to know the bus speed. If for example you have a Socket A motherboard for AMD processors you cant go and buy an Intel P4. The relation between CPU speed and Bus speed is a multiplier, eg if you have a 1000MHz processor with a 100MHz bus then it will use a multiplier of 10, if the bus speed of the CPU was 133MHz then the multiplier would be 7.5. Your motherboard may be able to switch from 100 to 133 by either a jumper, dip switch or a setting in the BIOS.
Hood