Heh well you can forget 4ghz on stock cooling. In fact, with stock cooling your OCing options are very limited. IMO, at most if you have 1600mhz RAM, set your Base Clock to 160 and set the RAM timings to their ratings and leave everything else on auto and see what kind of temps you're getting.
If you decide to upgrade you cooler (an absolute must for OCing) then some rough voltage settings you might use are:
175 base clock, turbo on
1.31Vcore
1.21VTT
200 base clock, turbo off, EIST off, 20x multiplier
1.35Vcore
1.25VTT
LLC on. If you have Vdroop, you'll need more Vcore (for both these settings)
Anyway, OCing all comes down to the user doing it. How much can you get? Mobos and CPUs have their own quirks. Some are better than others. The point is, try it for yourself after reading some how-tos. Just make a small jump, test it... if everything seems ok, increase it a bit more. OCing should be a fun or at least interesting process. It's not like setting some default values (ie: for this, input this. For that, input that).
Also, Tom's did an i5 750 review on OCing and efficiency you should check out. Goes into some detail and also has voltages posted so there will be some good info for you there. (Oh, that's what matto17secs linked
)