radio battiers
Mah is only a capacity rating...the higher the number, the more "juice" it'll hold. The voltage output is what you need to be concerned with. However, be careful when you charge it. Keep the charge rate at 1amp. Any higher will only shorten the life of the batteries, as well as your run time.
If you're running an electric truck, then you charge around 3-5amp (depending on the quality of the batteries). The higher the amp charge, the more "punch" the batteries will have, but they will discharge quicker.
Being that you don't need to worry about "punch" with the RX battery, keeping the charge rate at 1amp will insure you get a long run time out of it.
I have 1600Mah AA Sanyo's from Ballistic Batteries and I can go ALL day with my Tx and the voltage will barely drop! Good batteries make a world of difference.