mAh, a measure of battery capacity, stands for milliamp-hours - current times duration. Weight is roughly proportional to capacity.

AR, AE, and AAU sound like different types of NiCd cell. They will have different weights and are capable of supplying different amounts of current. Of course, a light battery that supplies lots of power is going to be expensive.

The rating on the speed control is a maximum. (Think of it like a fuse - a very expensive fuse! [img]i/expressions/face-icon-small-shocked.gif[/img] ). The motor, gear ratio (if there is a gearbox), and propeller determine how much current is drawn from the battery. It's OK to use that speed controller with a smaller motor such as a speed 280 (max ~5 amps) or speed 300 (max ~10 amps).

Search for EZone magazine and look at the FAQ - they explain a lot of this stuff.