8800 GT

core 701, shader 1716, memory 921 </snark>

Overclocking is a process of trial and error, you are going to have to increase the speed until you start getting errors and you won't know what your card will do till you hit it's max. Read some tutorials and don't go too fast.
That said, 770 core and 1020 memory should be a safe guess, depending on your cooling.

Download and install Rivatuner. (google is your friend)
Read the driections on how to use it. (google is your friend, again)
The core and shader clocks should locked together, keep them that way.
Start by raising the core a little at a time, and running for a while, checking your temps and artifacting. Keep going until you start either getting too warm, or you start having display problems and artifacts.
Then, you can start tweaking the memory upwards, but the memory will not overclock nearly as far as the core and shader clocks did. It is pretty touchy, so don't expect to get too far with it.