Easier to overclock Dual or Quad?

While the process is the same, you can't expect as large of overclocks on quad cores. What motherboard is in the 3 year old computer? Some boards are really garbage for overclocking.

It depends a lot on which model you want to get. Intel's C2D and C2Q are basically the same thing, the quads just have two C2D per chip. Some models of each are known to overclock really well, while others don't do so well. The i7s overclock better than almost anything else around, and part of the reason the Q6600 remains popular is because of its overclocking ability. The Q8xxx series does not overclock all that well, but the Q9x50 chips all seem to be good overclockers. Just do a little research and its easy to find the best chips to overclock in both dual and quad core, and you can expect to gain about the same one each overall.

Built a gaming rig for a friend, running a Q8200 "R0" stepping 2.33@3.13ghz. More voltage is required to gain to near the 400 fsb and better cooling is required.

Quads are less overclockable then the Duo's is what I've found so far.

Multiplier on intels CPU aren't locked?

Thanks for anwsers/advices!
Actually, I have an intel core 2 on one of my PC so I'll probably OC it firt before doing it on a new PC (if it works, i'll probably get a new mobo... i have to see if it can work with the actual build anyway).
I was wondering what could make my OC screw up. I consider that if I follow a guide (like some on THW) I shouldn't have any trouble =O
P.S. - To OC a CPU you have to increase the voltage so it increases the intern resistance and produces more heat. Is that right?

Sorry, my bad. But on tigerdirect.com they say that the unlocked multiplier isn't active.

you could have 2 of the same boards and 2 of the same cpu and still have diff oc settings. every chip is a little different. the trick is to single things out. dont touch voltage and just start slowly notching up the clock speed until u have bluescreens. from that point u start messing with upping voltages and ram, etc.

I have an E5200 running at 3.75 GHz. in an old P35 motherboard, average cooler. Just can't get it higher with any stability.
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Overclocking since 1978 - Z80 (TRS-80) from 1.77 MHz to 2.01 MHz

I don't do high OCs, but a free 10% is just that, free. My expectation would be that you would run into heat problems a lot sooner on a quad. That isn't saying you can't go as high, it just means you'll have to do a better job of cooling.