Use rivatuner, raise the core clk (originally in 600 MHz) in small steps and use atitool to tax the GPU and check for stabillity and temps, if needed increase the fan RPM, stop when you get artifacts in atitool or when the temp gets to 88 ºC and thake a step back.
Then raise the VRAM speed (originally in 900 MHz) in small steps and repeat the procces above, usually toy can get somewhere arround 650-700 MHz core and 950 MHz RAM.
The shader clk gets raised along with the core clk, both rivatuner and atitool are simple to use, try google if you need instructions to use the software.
These cards are fairly simple, just basically the Core bus and Memory bus to worry about. Like TheBlade said before me, just go in small steps. Mine runs fine at 675 core and 975 memory but it varies per card. I'm not that knowledgeable about overclocking yet these are simple even for me, make sure to test for stability though. If you're lazy you can even use Nvidia's programs to do it, though from what I hear they're not the best..
+1 To use rivatuner, and also use it to up your fan speeds. Many 8800GTs have fixed fan speeds, as low as 29%, which aren't adequate to cool the cards at stock speeds, let alone overclocked! Google for a Rivatuner guide and it will show you how to set it up so that it controls the fan speed based on temperature.