F@H hammers it pretty damn well but there are a few programs that will test the true stability of a good overclock...
Prime95 is probably the most common program used for stability testing and allows for application controlled cpu affinity which is nice for us dual core users.
OCCT is another program with a built-in torture test program that really puts some hardcore strain on the CPU and get's it cooking nicely.
SuperPi 32mb test is probably the weekest of the three mentioned but is a good place to start. If you can pass a 32mb SuperPi then you know you have a leg to stand on for total stability with Prime95.