I use msi afterburner, I would start with +75mhz on the core clocks then increase it in 25mhz increments, then test using either a game that will push the gpu to 100% usage or a benchmark like unigene heaven or something. If its stable increase it another 25mhz and retest until it freezes, blue screens or hard crashes then back off your clocks a bit test agin and find a stable number.
After you find a stable core clock I would stat OCing memory, start with a +100mhz, increase it in say 40mhz increments until it becomes unstable, when memory becomes unstable it will sally start artifacting, you will see odd patters and colors on your screen that are not supposed to be there, when you see that back off the memory clocks and find...