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.
currently my maxed tested and successful overclock on gpu is about 728mhz using evga precision. the next step up at 732mhz it locks up Real-time HDR IBL with 16AA enabled after a little while (i use this program to burn test my overclocks, works real nice just leave it in window mode, not maximized but stretched to fill up at least half your screen and enable 16x AA. it will push your card to its highest temp it will reach quickly). temp gets to about 77 but fan is on auto so it keeps it down.
edit: final overclock stable and tested is 728mhz gpu / 1119 memory. gained about 7fps average in crysis timedemo benchmark at high/very high settings (0 AA). temps stay at 78C or lower. stock frequency that i had was 678 and 970 gpu/memory which is overclocked by the manufacturer.