You would be lucky to get atleast 50 to 70MHz more out of that if you're lucky. The older iMacs have the CPU on the backside of the logicboard and basicly uses the frame of the case as the heatsink. Not bad but the heat onlt travels so far. Now if you can cut away the frame so you can fit a standard size (2 to 3 inch) tall heatsink it will add some extra cooling. Other then that if you plan to try majorly overclocking it I would use a diffrent case.
To live is to let die