The Central Processing Unit and the main memory must both use the same Front Side Bus speed. The FSB connects the main memory and the CPU to each other and is used to provide access to other components in the computer.
Changing the speed of the FSB alters the speed of the CPU. The clock speed of the CPU is calculated using the speed of the FSB and a multiplier. In many cases it is possible to increase the speed of the CPU above its advertised speed by increasing the speed of the FSB. This practice is known as overclocking.