All fellow overclockers I overclocked mine stock 8800GT(512) to
Core: 700 MHZ
can it go any further without artifacts all over the screen and also rock stable while playing Rainbox 6 Vegas 2
G92 8800gts can reliably oc to 750mhz core, some went as far as 800mhz fully stable and without artifacts.
If your 8800gt use the stock single slot cooler, it'll hold oc back due to heat problems. Still, it should go further than 700.