How to Overcklock Your Graphic Card.

You will get about 10-15% more performance for free. 
The only bad thing is that the temperature will increase and life of the hardware will decrease. 

I know this work whit NVIDIA but I think it works whit ATI to. 

Start to download RivaTuner, Furmark and GPU-Z. Install the programs and follow the instructions.

Download GPU-Z: http://www.techpowerup.com/gpuz/
Download Fur mark: http://www.ozone3d.net/benchmarks/fur/
Download Riva Tuner: http://www.guru3d.com/index.php?page=rivatuner

Start Riva Tuner. At the moment Riva tuner doesn't support the latest NVIDIA drivers. 
This is done under the tab "Power User". Navigate there and expand the list "System". A bit down in this list, a parameter called "Force Driver Version". This value will be changed to 18585, ie the driver's version number. Save the settings, click "OK". Restart the program.
Always keep an eye for the new version of Riva Tuner. 

Although the program will still say that the drivers are not supported, but we can access the overclocking menu. Click on the small arrow to the right under Driver Settings. In the four icons that pup up you will choose the one to the left, that looks like an graphic card. 

Click on "detect now" in the window. 

The upper lever controls graphics processor (GPU) clock frequency, the middle takes care of the stream processors and the bottom changes the memory clock. Move the slider to the right to increase the frequency. 

Now is the time to trim the clock frequencies. Start Fur mark and GPU-Z. The first program is used to check if the clock frequency is working and what temperature the graphics card reaches, and the other to see which clock frequencies used at the moment.

With Fur mark and GPU-Z running, we return to over clocking tab in Riva Tuner and start raising the clock frequencies. It is easiest to start with only the GPU and then take care of the stream processors and memory by hand. Raise the clock gradually by 10-15 MHz at a time. Fur mark ensures that the graphics card is under full load, which will appear when the temperatures shoot away.

Depending on the graphics card is used; sooner or later encounter a limit where graphics error occurs. This is usually seen in Fur mark as small dots or nails projecting from the hairy ball. When this occurs, lower the clock rate slightly and try again.

When you know the clock frequency GPU is capable of, just apply the same method on the stream processors and graphics memory, with small incremental increases in time. Should your computer freeze because of too high over clocking, just reboot, then the default settings will be loaded. 

Let Fur mark runing a moment to make sure that the clock frequencies are stable. Between 45 minutes and two hours will do it. When you are satisfied, it is possible to let the Riva Tuner apply the settings each time Windows boots. To do this, tick the option "Apply Over clocking at Windows startup" in the over clocking menu.

Now you are done. You can see how much the performance have increased by running 3Dmark before and after.