Got a lot of experience in overclocing generally, but not specifically the E6600....That said, 3.0GHz should be an easily acheivable target if you motherboard and BIOS allow for the right settings.
In any case, there is no way of saying what core voltage you will need for your processor. They are all different, and by a large degree.
A lot of what you need to do depends on the motherboard, RAM and cooling you have - if you can post back some details I'll gladly give you some pointers. Need to know:
Motherboard make and model
BIOS revision if possible
What CPU cooler yopu are using
What RAM (make, model)
Processor VID if possible (use CPU-Z to get this information click here)
It would help if you could say how much experience you have in BIOS adjustments.
Overlclocking is not something you should jump straight into, but you can get solid results if you take your time.