The actual part of overclocking your graphics card is fairly simple.
Most manufacturers have a tweaking tool in the drivers to change core and memory clock.
If your manufacturer has not supplied you with the provision of changing the clock speeds of your card it is advised that you use software like Powerstrip. Powerstrip is an extremely useful tool when you are overclocking your graphics card since you can get a detailed description of what your card is doing. Furthermore you can do many things to your card that you wouldn't be able to do with other software. First of all you should remember to go slowly maybe 5Mhz at a time is very much advised. However you must constantly check for instances of improperly rendered scenes shabby frame rates.