Download MSI AfterBurner or EVGA Precision Tool, Bump up Core clock by 50 MHz, Memory Clock by 70 MHz, Shader clock by 50 MHz... then test stability (Play games, Watch HD Movies....) if no problems happen increase Core clock , Shader & Memory clock by (xx MHz) and then test stability....
you can do this over and over until you are unstable (Crashes, Freezes, BSOD,...) Thus you know the limits of your GPU.
Note 1 : check temps after each overclock you made 85C is the maximum acceptable temp.
Note 2 : if instability happens (crashes and so) you can bump up the voltage of the card and this will make you stable, and let you overclock higher and higher, but i don't recommend it cause if you mess up with voltage you can kill your GPU instantly.
Besides, you can achieve higher clocks with stock voltages, Good Luck.