First things first, overclocking is never an exact science and your results will differ from everyone else's. That said, I have almost the exact same components, so here are my results to give you a head start:
4.0Ghz - 1.20V
4.2Ghz - 1.22V
4.4Ghz - 1.245V
How far you can push the overclock also depends on the ambient temperature. If it's 30C, the computer will be sucking in 30C air, so you immediately have less overclocking headroom than if the ambient temperature was 20C.