If you want more speed, you can have it, but will pay dearly for it in decreased operational life. That isn’t to say don’t do it, keep in mind that most hobby robots are run in short bursts infrequently, and you can replace the motors (Pololu sells them for $1.99). It looks like somewhere between 4 and 5 volts gives you the best payoff, but beyond that you’re really doing some damage. Speaking of damage, I have a nice little motor graveyard going on my desk:
Since this article was first written, Pololu has started selling a similar 130-size motor ($1.79) that is designed for operation at higher voltages (3 – 12 V) and draws under an amp when stalled at 6 V. These motors can be used to replace the Mabuchi FA-130 motors in Tamiya gearboxes for lower-current or higher-voltage applications. These motors also produce less electrical noise than the Mabuchi FA-130 motors.