Testing and Adjusting
Now that you have your hardware overclocked, you should test it to make sure it is stable. For this, you need a 3D benchmark. There are programs such as 3DMark which are built specifically for this, but many popular 3D games, such as Crysis, also include 3D benchmarks which will work equally well. Open your program and let it run through a cycle of testing. You're looking for any sign of corruption. If textures look werid, if geometry seems to be out of whack, or if at any point the entire screen flickers, changes colors, or goes black, then you need to back off of your overclock. Obviously, a benchmark which crashes or freezes is also a good indication that you've reached the highest overclock your video card can tolerate. The most subtle sign of problems are artifacts; dots one pixel in size that don't get redrawn as the GPU heats up, so they stay the same colour and stand out from the surrounding pixels.
Getting the most out of your over-clock essentially involves going through the cycle of adjusting the settings and running your benchmark until you find the highest core and memory clocks you can run without the benchmark crashing or corrupting. As said, I recommend moving up in 10mhz steps, but if you feel impatient, you can increase the over-clock by more with each step. Just be careful, because the larger steps you take, the greater the risk of damaging your hardware. Once you've found the highest overclock you can reach without crashing the benchmark, it is time to stress test. Doing this basically means running the benchmark over and over again, but you can also stress test by running a demanding 3D program, like a new video game. If you run into corruption or crashes, even after a few hours, then your overclock isn't stable.
Once you've found your highest stable overclock, you may still be able to squeeze more out of your card. Go back to the Low Level System Tweaks, then click on the Fan tab. This opens a menu with options that allow you to change the speed of your fan. The higher the fan speed, the cooler your video card will run, and this may allow you to run the card at higher speed than you could before. However, increasing the fan speed will also make it nosier and may shorten its lifespan, so you'll need to find a compromise between speed and noise. You'll also very likely reach a point where increasing the system fan has no effect on how high you can overclock your card.