...and it will depend if you are overclocking and by how far. shel0139 is correct - the stock cooler is ok - without overclocking. The Core2Quad Extremes will run up to 100C before the over temp kicks in and throttles them back. A good max temp would be 80-90C - but that dpends on your case getting rid of that heat fast...
I would overclock my C2Q until I hit a constant 80-85C on a benchmark.
Also the stock cooler is a little slow on spinning up and typically waits until 80-90C before spinning up (keep noise levels down).
An aftermarket fan will dissipate the heat faster and run quieter ...