The primary two stickies are:
HOWTO: Overclock C2Q (Quads) and C2D (Duals) - Guide v1.6.1 http://www.tomshardware.com/forum/240001-29-howto-overclock-quads-duals-guide
to know something about thermal management or you can fry your CPU. It's actually kind of difficult to fry a modern CPU, but it is possible.
Core 2 Quad and Duo Temperature Guide http://www.tomshardware.com/forum/221745-29-core-quad-temperature-guide
I have Q9550 and Q6600's running at 3.6 GHz and an E5200 running at 3.78 GHz.. I do not exceed Intel's recommended max voltage and temps. Well, except for the E5200. I pushed vcore to 1.50 volts. No joy.
I am brickwalled at 3.78 GHz.
----------Overclocking since 1978 - Z80 (TRS-80) from 1.77 MHz to 2.01 MHz