You do not want to boost the voltage. USB devices work at 5V. The iPad will charge at the standard 500mw, but will only show charging with 1000mw. In the future, more and more devices will be rated for the higher current, but until then, the best bet is to turn the iPad off while charging from a traditional port. It will be far easier to get additional chargers from the aftermarket vendors than to sweat boosting the output of any computer.