If you look up reviews for the CPU you are buying which have an overclocking test in it, then that will give you a rough guide to what other have been able to get out of their CPU speed wise, though as was touched on earlier, you mite not be able to get that out of the CPU yourself. Although the spec and model of the cpu are exactly the same (within each edition of course), the manufacturing process does not allow for each CPU to be a totally exact copy of each other, if that makes sense? (i think this may be what the100thmonkey was trying to say?)

Then obviously you'd have to take into consideration things like motherboard etc.

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