You get much less of a performance increase out of GPU overclocking than you do from CPU overclocking. I'd say its around a 3-4 increase in FPS for a ~10% overclock. That being said, you may as well overclock, there is almost risk involved with GPU overclocking.
To overclock, I would download RivaTuner and furmark. Run furmark in the background while you slowly increase clock by ~5Mhz. Make sure no artifacts start to appear then increase it another 5Mhz. Once your computer crashes or a lot of artifacts start to appear, back the clock down ~10-15Mhz and let furmark run for another 15 mins or so, making sure the temperatures stay within reason and your computer doesn't crash again.