Core 2 Quad and Duo Temperature Guide http://www.tomshardware.com/forum/221745-29-core-quad-temperature-guide
No point in overclocking if thermals kill the CPU.
And google "<your motherboard> overclock" to see if there is a guide for your particular motherboard.
And google something like "overclock stress test" or "overclock stability".
Lastly, remember these are guides
, not cookbooks.
There's just so much variation in solid state electronics that settings that work for me may not work for you.
And after you have been through the guides, come back with specific questions. We will be glad to help.