I wouldn't overclock your CPU from a OS utility, that sort of thing is best done from the BIOS.
That said, have you read the Intel Core i5 2500K/ Intel Core i7 2600K overclocking guide here on Tom's? It's a good place to start. http://www.tomshardware.com/forum/265056-29-2600k-2500k-overclocking-guide
That said, the best way to overclock in my opinion is to overclock in your BIOS, try increasing the multiplier by multiples of 1 (34, 35, 36, 37, etc....) and running Prime 95 for 20 minutes. You should hit 4.2GHz without needing to bump the voltage. When you start to bump the volts, increase it by .05V at a time. With good cooling, my 2500K's should top out safely around 4.7-4.9GHz.
Make sure Turbo Boost and Speedstep are disabled as well.