#1: You cannot overclock proprietary boards from Dell. Not sure about HP, but I have never heard of overclocking a Dell board. Best you can do is replace the CPU with a faster one within the same model and stepping.
#2 -- you want to overclock your CPU? We would need to know what kind of processor you what to overclock.
In your signature, you mention a Dell xps 1647, which is a laptop with a i5 -- I assume that there is a different processor that you want to overclock. Which one is that?