All depends what card you have, but for the most part its the same. For nvidia cards just head to
Click Me and download your choice of unlocking tools. Run the programs (usually just registry editors) then go to where you would normally ajust your video card under display settings. There should be a new tab that allows you to ajust the core speed and memory speed. Ive never OCed an ATI card, but im sure it has a similar process. I have a GF4 Ti4200 OCed to 290/590 and it runs rock steady, and added 1500 points to my 3dmark bench. With nvidia cards its easy to find stable settings as well. It tests the setting for a few secs and if its stable it will let you keep the settings. Good luck and try not to fry ur card