The 8800gts 512, G92 cards will overclock very easily.
Download Riva Tuner ^.
Leave your memory clock alone. Keep the shader and core clocks locked together. You should be able to push the core to 750-775mhz with the stock cooling. My BFG 8800gts will run without any problem up to about 785mhz with the stock cooler, and no fan tweaked settings.
The memory though is tricky, it will not stand nearly as much.
Get your best core speed first, the you can try bumping up the memory speed a little at a time.