The Tamiya motors are intended for 3 V operation, but they definitely work at higher voltages. The only down side is that using a higher voltage shortens their usable lifetime. One customer of ours did a fairly detailed study to see how voltage affected the lifetime of the motor used in the Tamiya gearboxes. You can find his results here:http://www.pololu.com/docs/0J11/all
Another thing worth noting is that many popular, inexpensive H-bridge ICs (e.g. the L298N
) have voltage drops between the input voltage and the output voltage, and the drops can be as large as 3 V at 1 A. What this means is that if you use a 6 V power supply with something like the L298N motor driver, you would only have around 3 - 4 V on your motors. Your best best might be to use something like four NiMH batteries as your power supply, which would give you a voltage of around 5.5 - 4.5 V as they go from fully charged to depleted.
Lastly, the Tamiya motors are very noisy (electrically), so make sure you solder caps across the motor terminals.