For 8800GTS:
1) Load RivaTuner 2.08
2) Load ATi Tool
3) Start Artifact Test in ATi Tool
4) Unlink GPU clock and Shaders in RivaTuner
5) Begin increasing GPU clock speed in 10Mhz Jumps until ATi tool starts to detect artifacts, at which point reduce clock speed 10Mhz and leave to test for 10mins
6) When artifact free GPU clock has been found begin overclocking the shader clock in 10Mhz jumps and repeat artifact testing procedure used in step 5
7) When artifact free GPU and Shader clock has been found begin overclocking the memory, repeating artifact testing procedure outlined in step 5
8 ) When all 3 clocks have been maxed leave ATi Tool to undertake 30mins of Artifact Testing, if at the end of 30mins no artifacts have been detected overclock is deemed successful.