Download coolbit 2.0 from guru3d.com install that registry tweak & U can overclock from the NVIDIA control panel
The 6800GS has a temperature sensor; it can be monitored from the GFX Settings panel itself.
First simply install coolbits, by double clicking it,& click on detect optimal frequency, once it sets, play some games for long run, 1 hr or 2 just to test the stability. Then increase the memory & core clock speed again, manually with 5 MHz increments, until u get lockups or reboots while playing games, in which case, decrees the speed again to last good clock value. Overclocking is dangerous so be careful while doing this... & don't blame us for ruining it. In case your GPU reaches 65C then don't increase the clock speed further