You haven't specified WHY you wish to overclock your CPU.
If it's for games, you should run the Task Manager in the background (CTRL-ALT-DEL) and monitor your CPU to see if it's reaching 100%. If not you are limited by graphics.
This test was easy for single core, but it's slightly more complicated for multi-core. Basically if NO core/thread reaches 100% then you have no reason to overclock your CPU. If one does, it's not certain as it depends on whether the others pick up the slack (how the game is programmed).
I don't recommend overclocking unless you know you have a reason. I overclock my X2-4800+ by 20%.
I first drop down the RAM timings to 166MHz (333MHz dual). I then bring up the CPU timings. If I don't drop the RAM first then my RAM becomes unstable. I'm not being very specific here.
There's lots of info on the net. It depends on the CPU as the architecture keeps changing.
You should just look up your CPU specifically, but again don't do it without a reason.