To OC you must download a OC utility like coolbits, Rivatuner, or Powerstrip. Do a google on any of those, and be sure to educate yourself before OC like crazy.
Install it, and clock it. On step at the time. + 5-10MHz. Find max Core freq, then max RAM freq. ANY progress check in benchmarks and 3d bloodsuckingthirsty4ultraGPUs games, such as D3, HL2, CSS, FarCry. When it will start to "lag" or you will see artifacts, you need to lower your clock frequencys.