How to OverClock ?

MB: ASUS P5N32-E SLI LGA 775 NVIDIA nForce 680i SLI ATX Intel Motherboard
CPU: Intel Core 2 Quad Q9550 Yorkfield 2.83GHz 12MB L2 Cache LGA 775 95W Quad-Core Desktop Processor
Video: ASUS ENGTX295/2DI/1792MD3/A GeForce GTX 295 1792MB 896 (448 x 2)-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
RAM: Corasir 2x2GB 800MHz DDR2-SDRAM CM2X2048-6400C5DHX 400.0 MHz (PC2-6400)

That are the specs.

I have that same processor running at 3.9Ghz. I also have 1150Mhz RAM which gives me a wider range of FSB numbers. Actually...I have a P5N motherboard as well.
I don't have my computer with me atm, though, since I'm in Iraq, but I'll try to remember what I had going.

First, power on the computer and enter the BIOS screen.

FSB-440~
core multiplier-9
(or try FSB 460, core 8.5)
Vcore-1.45(after getting it stable, if you do, lower the voltage as low as you can while maintaining stabilty)
The RAM speed is difficult, as we don't share RAM. There should be speeds in there you can choose. I don't remember what the option is called. Anyway, set the voltage to 1.9v, and try it at about 900Mhz. Keep in mind, the speed of the RAM depends on the speed of the FSB.
Good luck, and keep an eye on your temperatures. Don't let the cores stay above 75c.

I forgot. Turn off A.I. overclock, or anything that says 'Turbo' in the BIOS.

stingstang, I will bet you that your your P5N motherboard has a 750i/780i/790i chipset, not a first gen 680i.

lamah, you are lucky the the Q9550 even works with a 680i motherboard. My eVGA 680i motherboard does not work with a Q9550. That's why I shifted to Gigabyte motherboards.

This should be your first stop.
Core2 Overclocking Guide (generic guide)
http://www.tomshardware.co.uk/forum/259899-11-core-over...

680i overclocking guide
http://pc.ign.com/articles/747/747606p3.html
This is pretty good except for the section about memory. Try the "Linked" setting first. If that is not stable, try the Unlinked" setting. Then set the memory clock to twice the FSB frequency. Do not try to overclock your memory. The possibility of system instability is not worth the minimal performance gains. We discuss that here:
http://www.tomshardware.com/forum/251715-29-ratio-myth

You are better off working up from the stock settings than you are working down from stingstang's settings. You are better off not trying to cookbook the settings. Each CPU chip is a little different.
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Overclocking since 1978 - Z80 (TRS-80) from 1.77 MHz to 2.01 MHz