You can check your overclocking results using the 3dMark2001 benchmark. I would recommend only incrementing in small steps. When you have overclocked it too much, 3dMark2001 may lock up, or you may start getting graphics glitches, at this point I would knock the speed back again a small bit. I would run the benchmark again and again, just to stress test the graphics card to make sure it is stable. You may consider getting an additional fan and bolt it to your graphics card heatsink, or place a temperature probe in there also so you can keep track of the operating temperature.