google "coobits tutorial" it will provide you more information than we could explain in this forum. Also, the actual increments you should use in overclocking will vary across different cards and models. I suggest you start much smaller than suggested above until you have some experience or know that the 6800GS is a strong overclocker.
Quoted from tweakguides.com:
The whole overclocking procedure goes something like this:
1. Select a component to overclock (Core or Memory).
2. Increase its clock speed by a small increment, e.g. 5MHz.
3. Run a game or a benchmark for a short period (See Benchmarking & Stress Testing section below), testing to see if you get any crashes/freezes, stuttering, or 'Artifacts' (small graphical anomalies, such as white dots, strange color blocks or flickering textures).
4. Repeat steps 2 and 3, until you experience a problem - this means you've reached the current limit of your overclock for that component.
5. Reduce the clock speed by at least 5MHz to provide safe headroom.
6. Repeat the entire procedure for the other component (Core or Memory).
7. Once you believe you have both a stable and fast overclock, do a much longer run of benchmarking and/or play some games for a longer period of time (e.g. 2 hours), paying particular attention to any small artifacts which may appear, indicating the need to reduce the overclock ever so slightly and/or increase cooling.
Source:
http://www.tweakguides.com/NVFORCE_11.html