geforce 8800 gtx

I like to use Evga precision |MG| EV GA Precision 1.9.6 Download This tool will allow you to control your fan speed and adjust your clock speeds.
No one can tell you what your card will accept as a stable overclock as exact same cards will clock differently. Their tested to preform at the advertised speed some chips will clock higher when others will just clock at the advertised speed. Just depends on how lucky you were when you purchased your card if you received one that can handle a decent overclock.
I like to use Evga precision because you can link your core clock with your memory clock and increase them a little at a time, then click test to see if it will handle the increase.
Once you've found it's limit and it won't pass the test back it down till it will. Then to be on the safe side I back it down a little more for stability. You'll also want to keep an eye on your temps, and adjust the fan speed accordingly.
You can also adjust them unlinked since the core clock and memory clock may adjust better separately. Still remember to use the test function as described above.
That's my 2 cent worth. Fabe
Also under the tool icon you can see all the program monitoring tools, OSD and FPS. As well as plenty of other options.