720mAh NiMH cells are normally not good enough for Speed 400 applications. Now, I did run the same gearbox and prop, using a Speed 400 motor, and the current is down around 8 Amps. That's on the ragged edge of the 720mAh NiMH's capabilities. It likes 7 Amps maximum much better. If you limit your full-throttle time to short bursts, you can probably get away with that combination, though.

If it were me, I'd consider using the Wattage 5/4AAA 800mAh 8-cell NiMH pack, found at http://www.hobbypeople.net. This pack is only 25% heavier than a 720mAh AAA pack, and still signifigantly lighter than a 2/3A pack of Sanyo 600AR or HECELLs. It can handle up to 10 Amps, and provides 10% more capacity.