<- GA-P35-DS3R 2.0, here, with a bum E6600 (took a chance on a side-grade, and it won't even hit 3.1GHz! Came out of a flaky mobo, though, so I think it might have been overvolted and OCed, already).
Basically, transistors switch faster at higher voltages. They follow a clock signal, in the CPU. So, once you reach their limit at a certain signal rate, they need higher voltage applied to run at a faster rate. However, of course, some might just not work any faster than some speed, and you end up stuck. Increasing voltage, however, also increase power consumption exponentially, increasing speed increases power consumption linearly,
and increased operating temperature leads to higher power consumption, too, but I can't recall off the top of head the relationship. So, what appear to be small voltage changes, like a few hundredths of a volt, can pan out to be quite a bit, in practice.
An E6550 runs at 2.33GHz stock. That's 7*333MHz (1333MHz marketing FSB, as it transfers data 4 times each cycle). You can choose a lower multiplier (probably only 6x, though), but no higher than 7. So, to run it faster, you have to increase the FSB speed. The chipset itself will probably cap out somewhere around 450-475MHz, at least at sane voltages, but RAM usually gets you, first, so you shouldn't end up having to overvolt the chipset. Most folks with 4GHz CPUs planned out their overclocking to start with, and bought 1066 or 1200MHz rated RAM, instead of the more common 667MHz and 800MHz varieties.
To get higher speed, you will need to have RAM that can run at at least 2x the FSB speed. So, for stock, that's DDR2-667. For 2.8GHz, DDR2-800, for 3GHz, DDR2-866, and so on. Change the memory multiplier to 2.00 (it's literally that, in the list), right off the bat. You can go to 2.8GHz, as it is, without overclocking your RAM.
Chances are very good that you can run 2.8GHz (400*7) with no more changes than the memory multiplier set to 2.00, and FSB to 400MHz. Most C2Ds could get to about 3GHz on stock volts. Then, assuming it runs fine, run some stress testing software, like Prime95, and OCCT.
As you start to go higher, your RAM can become a bottleneck, since it's only rated to 800MHz (400MHz FSB x2). So, you might need to loosen the timings (hit either ctrl-f1 or ctrl-f7 in the main BIOS menu screen, to enable RAM timing options. You'll see it flash. I haven't done it in awhile, so I've forgotten which f-key it is). That is to say, if yours is set to 5-5-5 (common for the time), change it to 6-6-6, and see how that goes. Often, you could get RAM up to 900+MHz just by that. The DRAM DLL setting is worth changing, too. I forget what it is, now, but IIRC, neither setting is good or bad, but worth changing and trying again if one fails. Also, try increasing the RAM and CPU voltage, if it's not passing tests and/or is unstable, and/or you get WHEA errors (
check out this utility, to make it easier). Aside: your CPU has the means to detect certain classes of hardware errors. Some are correctable, but you should not see more than a handful during the entire CPU's lifetime, so getting even
one shortly after making the CPU faster via overclock is a sign of a bad OC, even if things appear stable.
When an OC fails to boot, the BIOS will reboot with stock CPU settings, but show you what the failed settings were, in that blank space to the right.
That should get you started. If you are using the retail Intel CPU cooler, don't increase the stock CPU voltage much. You can probably get away with +0.05V, but any more, and I wouldn't trust it.
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