The motors in the Tamiya Dual Motor GearBox are an odd bunch. They are low voltage (3V) and relatively high current (over 2A stall current). So you have a couple problems here.
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These motors won't live long running over 3V, and you are driving them at 4.8V.
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The SN754410 is only rated for 1A current, and your motors are drawing over twice that. (They also only operate down to a minimum of 4.5V, so just aren't suited to work with the Tamiya motors at all)
You have a couple options to remedy these problems.
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There is a pololu motor driver that was designed to work with the Tamiya motors, able to operate at 3V and handle the high currents.
http://www.robotshop.com/pololu-low-voltage-serial-controller-2.html -
There are replacement motors that will work with the Tamiya gearbox that operate at higher voltages and lower currents.
This motor will work fine with the SN754410 H-Bridge
http://www.robotshop.com/solarbotics-regular-motor-3.html