1)A simple way to overclock your GFX would be to use the ATItools.Go google for it and install.
2)In the second step, you increase the memory speed and the core speed slowly(preferably in 10MHZ increments) until you start seeing artifacts appearing on your screen like
these 3)Then proceed to decrease the memory speed and the core speed one at each time until the artifacts disappear.Now this is halfway mark.The clockspeeds that you have now may be unstable during gaming so you have to run benchmarks
4)Try downloading 3Dmark03 anhat 3Dmark does not crash or show artifacts during the test.d 3DMark05 and run it in loops.The rough gauge should be 2-3 hours, make sure t
If it fails the test repeat 3) until you have a stable clockspeed.
Disclaimer
o it at your own risk and no one is responsible.Some Distros may not accept warranty due to overclock-induced damage