All processors are by default down-clocked.. so to say. Look on the www for overclocking your specific processor, chances are you can find how-to, the point where it runs stable and the cooling that goes with it.
Later 45nm processors almost don’t overheat and can be clocked a lot further then their factory settings. But truth is.. you will find no or hardly any gain in speed. And the frequency don’t mean shit these days.
Core2Duo’s should still run almost every system pretty snappy and 2GB of memory is plenty. If you feel the system is slowing down, look for a faster harddrive or write your current one with zero’s and do a fresh install to get that ‘the-day-you-got-it’-feeling. Overclocking, while pretty safe, should only be done with disposable cpu’s.