I've overclocked my P4 3.0 northwood to 3.4 without any problems running on a Corsair 200ex WC kit. I've heard that beyond 400mhz, I need to adjust my voltages. How would I do that? and what does it do?
I've found the quality of the RAM, as well as voltages for the RAM, to be infinitely more important than the vcore on a P4C setup. This is not to say that's it's worthless, but there's really no reason to play with voltages unless you're experiencing stability problems.
I'll second Raven's bluntness in that you need to be educated on what you're getting into (not just on voltages) before poking around in the BIOS. Here's a couple links to get you started:
Detailed BIOS Guide