I'll up voltages as much as 10% and that is all
You often don't have to go above the manufacture spec at all.
CPUs get bin'ed by voltage too.
Lets say the CPU core is rated for 0.85 to 1.5V (like my core2 Q6600 is)
CPUs that get binned to run at full speed down to 0.85 - 1.0V are supplied to laptop manufactures due to being really efficient.
1V-1.3V might be general desktops and 1.3v-1.5v are either rejects, or get their clock lowered until they will run ok at a lower voltage then sold as that. etc...
So if you get a CPU that's been "set" to run at 1.1V you can safely up the voltage to 1.4V (27%) and still be inside the spec.
However, what you do have to watch is the idle vs load voltage, it dips a lot at high current depending on how good your motherboard is.
My Q6600 is stable under load at 3.2ghz and 1.3V but i have the bios set to 1.45V to get 1.3V under load.
The CPU could go faster at 1.4V under load but not without exceeding the voltage spec when idling.
The top market overclocking motherboards have better voltage sense lines so this is less of an issue.