How to overclocked CPU?

I need to overclock my CPU from 700 Mhz up to about 1.2 ghz. Is this possible, keeping in mind that I am capable of attaining a really good liquid cooling system?
If so, please tell me how, to my shame, I know little about hardware, besides what the benchmark numbers mean, and stuff like that.

I wanna play HL2 dammit!

I got Half Life 2 to run on 750Hmz with a DX6 Card (KryoII 64MB), sure it was on the lowest setting and only Garry's Mod with that construct map because Half Life 2 took too long to start up (Too much info).
It got about around 25 to 30 fps.

I'm on a 2.4 Ghz with DX8/9(Fx5200 Ultra, It know it sucks) card now.