I've gotten bored and decided to finally get around to building myself a USB power injector for my 770. Thanks to Thoughtfix, I've got a firm grasp on what needs to be done and all that's missing is a bunch of knowledge with electronics. I'm not a complete dunce but I need some questions answered.

For whatever power source I use (I've still got my eye on a 9v battery), the voltage needs to be dropped to in the neighbourhood of 5 volts.

Some people have said to use a voltage regulator, some people have said to use a 5.3v zener diode. Which would be better? If the +0.3v from the diode would be dangerous, I could always throw a resistor in series after it. I also understand the regulator would eat up 2v defacto whereas the diode would only eat up 0.5v.

What happens if the voltage drops too low? If I feed the USB port 3v will it cause any damage to either the 770 or my peripheral? (ie an MP3 player)