In Powerstrip, go to
Advanced Options and then
Performance. That's where you find the overclocking selectors.
The problem with Powerstrip is that if you don't pay for it, your selections aren't saved and I guess now that I've said that, it occurs to me that I don't remember whether or not the oc'ing features are in the free version or not.
Tweakfiles is always a good place to look for something like an overclocking utility.
What kind of video card do you have?