If you really want to overclock make sure of a few things.
1. that the cpu gets the power it needs
2. That you have a cooler able to handle the extra heat generated
3. That the surface/contact area between the cooler and cpu is perfect
Fail any of those criteria and you could end up with a fried cpu or motherboard or similar.
My personal advise about overclocking is.... Don't...
Running them stock means they will perform as the cpu is rated for,
and will keep running stable until all the dust you forgot to clean out of your cooler/fan cuts off the airflow. (that thing is a dust magnet, air on a can is your best friend)
Almost anything in a PC can be overclocked these days,
so why does the factory not do this allready at the assembly lines?
Simple. They wish to sell stable products when possible.
What they do is usually test the cpu's, find their point of error, then throttle back a bit, this becomes the rating of the cpu.
In most overclocking guides you will see.
"increase step by step until the system becomes unstable or you see artifacts, then step it a down again a little"
I'd be scared to run my cpu or ram etc. "on the edge" changes of corrupt memory, file transfers, bus errors and more increase more and more the closer you get to that threshold.
The factory rating is usually (unless faulty or damaged during transport) within a safety margin.
Also don't forget that overclocking is a artform in itself, the pro overclockers out there know the exact memory timings, fsb/hypertransport, pci/agp, cpu voltage, and what brand, make/model/year/bios revision, and whatnot drivers and who knows what else to use to break those thresholds and still stay miraculously stable (most of the time anyway).
When I realized I could buy mid range (a bit below midrange even) cpu, and then in a year or two "upgrade" to a faster one.
(thinking AMD's AM2 dual core range whatever those might be, in fall 2007, only got a single core currently)
And pay less than half of what a overclocker would spend on a great cooler and fan solution and memory with proper timings to match the cpu's performance.
It is not worth overclocking just to get 5 more frames per second in this or than first person shooter game.
Nor is it worh to get a watercooling system (or similar noiseless system) unless you want a noiseless system or just want to impress people that is.
Remember, if something breaks...you'll end up buying a new cpu, or memory or something else. Was it really worth it? a overclocked cpu may work fine today, but a month or two later? Who knows?
And worse, data corruption, you may not always detect that until many weeks later. (depending on how much junk you got and what stuff get corrupted first obviously)
You definedly need to be obsessive about backups in that case.
You can only choose stability, or performance. Not both at the same time. And the old saying.. you get what you pay for
Links to some sites 
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