Overclocking your card can squeeze a decent amount of extra performance from your GPU, Still, before diving in, you should be warned of at least two things:
1. Overclocking may damage or shorten the lifespan of your GPU. You are pretty much running it above the factory-set specifications that are (ideally) balanced to provide stable performance over the GPU's entire lifetime. By upping the performance, you put the card under more stress and heat, which can damage or destroy vital components.
2. As a corollary to that, overclocking your GPU usually voids your warranty, as you're running the device under higher levels of stress than it was optimally designed for. It's not too difficult to overclock safely, but if you do something wrong and toast your GPU, don't expect it to be repaired or replaced.
If those two warnings aren't going to deter you from squeezing more performance out of your graphics hardware, then carry on!