So you've just spent the majority of your weekend tweaking voltages, multipliers and the bclk. You have gone through several blue screens, BIOS resets, and have finally managed to get into Windows—so far so good, no crashes, no stuttering.
As much as you may want to celebrate, you’re going to have to resist the urge as there is much to do before you can brag about that two fps advantage you’ve just obtained. To be sure that your system is stable, you need to push it to the limit.
There are many programs that will simulate a workload; most of them work equally well, but I prefer Intel’s XTU. If you have an AMD CPU you will need to use another program; AMD’s own “OverDrive” is a good place to start.
Intel’s Extreme Tuning Utility not only allows you to stress test and monitor your system, it also handles overclocking if you wish to stay out of the bios. Stress testing your memory or CPU is a one click process (if only stress testing ourselves was so easy), which makes it superior to some less intuitive solutions.
Generally stability can be assumed after a few successful passes with XTU or whichever program you are using; however, this isn’t always the case. You may still experience performance issues under certain programs and will need to adjust your overclock and see if the issue is resolved. Longer stress tests will reveal potential issues early on--it’s entirely up to you, I know I don’t like waiting around for hours to ‘prove’ stability.
Unstable overclocks cause crashes, stuttering and other performance issues. There really isn’t much to it, but stress testing is an important step to getting the most performance from your system.
For more extreme long term testing, you might want to try a program called Prime95. While the program was mainly created to find new Mersenne prime numbers, it's use as a stability test is popular due to its high intensity load on a PC's subsystems. If your PC can survive for 10 hours or more under Prime95's Torture Test, it's considered stable. Prime95 will work on any CPU.
Some folks will want to download 3DMark and run a looping benchmarks for several hours to test GPU stability. Or if you want, you can run both Prime95 and 3DMark at the same time—in a loop—although this seems extreme.
Once you past a several solid hours of system stability under the above conditions can you break out the Mountain Dew.