Overclocking the CPU
Overclocking the CPU can be the most rewarding, costly, difficult and easiest to overclock at the same time. CPU's are measured in clock cycles, and it is
classically determined by FSB*MULTIPLIER=CPU SPEED. All desktop CPU's to date use this basic concept, except for Athlon 64 (K8) based CPU's, which use a variation. A64's use the HTT*CPU MULTI to determine final memory controller & Core speed.
Most commonly used CPU stress testing, burn in, and stability testing programs:
When overclocking the CPU, some people prefer finding the maximum core speed; some prefer to max out the FSB first. While the FSB is the communication speed of the CPU and memory controller, some believe that the CPU has an effect on overall maximum FSB, but I have yet to see any solid evidence of the silicon limiting this. With the addition of the memory controller onto the CPU, the "maximum" memory controller speed may not longer stay the "maximum" stable operating speed because of the ability of the memory controller to keep up with the CPU speed at a 1:1 ratio, therefore overclocking the CPU speed overclocks the memory controller speed. This helps to reduce latency times, but may limit your overall overclock if your memory controller isn't capable of operating at the CPU's speed.
Voltage is necessary to be changed as well, to allow greater speeds. Basic CPU overclocking guide can be found
here