How to overclock Intel Core i5-2500K on Asrock Z68 Extreme4 Gen3?

The best method is to do it from the bios, you better download the realtemp and keep on checking the core temps when it will be stressing during testing, I believe that it will be not go more than 70c on sandy bridge, you better raise your cpu multiplier such that it reach the cpu speed which you would like to have it or to clock, do the same for the RAM. I would suggest that use xmp for RAM settings or even can set up manually, set the DRAM voltage which is been printed on the RAM. I think VTT voltage can be around 1.08-1.10v and CPU voltage around 1.2-1.25v for 4 GHz, but it is totally depended on CPU which you are using it. So when done with all this then you have overclocked the system.