You can use a program such as ATITool to overclock (OC) your card. Other programs that also work are RivaTuner, nTune, etc. I haven't kept up with these programs recently but I used to prefer ATITool. Using programs inside of the operating system to overclock your card is referred to as software overclocking. When you apply the overclock the first time, be sure to go back to the program and confirm that the clocks actually increased. For instance, in ATITool when you slide the bars up to OC and you click apply, the bar may go back to where it was and the program will not indicate the new clocks that you set. If that is the case for you then come back here and I will explain how to circumvent that (you can private message me if I don't answer you).
Alternatively, you can overclock your card by flashing the video card bios. This has the advantage of no programs needed and the settings are maintained across operating systems if you use more than one operating system. Under certain circumstances though, this method has the potential to immediately render your card useless. However, this is still my preferred method.
I would advise you to try the software OC. If it does not work then you will have to flash your card at least once and I will assist you.