Well I can't say you'd automatically get 100% if you overclock it, but as a reference I have a e6750 overclocked to 3.6 GHz and only on heavily threaded games (3 or more) is my CPU not enough. But that said, I'm hopping on the Ivy Bridge train next year because it's not enough for me.
If you get it to 3.0 GHz then you'll be good enough
The way to check is when you are playing a game and open your task manager and see if your CPU usage is at 100% or not. If it isn't then your GPU is being fed as much as it can handle.
Edit: Above is a bit of a generalization, but it's true most the time.