Overclocking performence

[umaza]

I have a Q8200 clocked at 2.33Ghz, if I overclock to 2.50Ghz or 3.00Ghz what amount of performance boost will there be?

I have an OEM system so I'm assuming I could only overclock through software, which program will be the best to overclock.

[Bositman]

Usually the speed increase is linear, so 10% clock speed increase, +10% speed (give or take).
No idea about software overclocking, I'm avoiding that since I can do it from my mobo much more safely and efficiently