RE: 4.8 and 6 volt Rx batteries
The above replies are correct, but what they aren't specifically saying is that you will get less flights from a 5-cell, 6 volt pack with the same capacity as a 4-cell, 4.8 volt pack. The power/energy used is related to current so you draw more energy from the battery to make your servos put out more torque at a faster speed. For this reason, you usually need a larger mil amp hour pack.
Also remember, your volt meter/battery checker will need to be re-set or changed for a 6-volt pack.