How good is a over clocked CPU?

[dieotherwise]

If a Pentium D 820 has been over clocked to 3.5GHz does it require to increase its vcore?

how good is a overclocked cpu? Is it going to run at the same lvl as it was before if one were to use it again in the stock speed? Assuming it is not damaged?

thank you

[Omsion]

Depends on the chip...take a look at the overclocking stickies.

An overclocked chip is as good as whatever you OCed it to - although not all applications have a 1-to-1 performance increase with CPU speed (aka increase clock speed 30%, performance increases 30%), many do and will see a major benefit. If you reset to default, it will perform the same as before.
And if it's damaged...well, lets just say damaged=dead in almost all cases. But it's really hard to kill a CPU these days, from built-in heat throttles and a greater tolerance for high voltages. You would really need to push the vcore to have a chance of killing your CPU.