Well, I would start by determining your current power requirements using your current battery capacity and runtime. Once you know that, you can estimate the amperage at any voltage, and then see if your batteries are up to it. This page may help with this:
http://scriptasylum.com/rc_speed/power.html
But a quick example; If you are getting 20 minutes of runtime on a 4s 5000mAh battery, and you want the same runtime and speed, you will need a 2s 10,000mAh battery on the appropriate motor.