Dual channel really speeds up data transfer through the RAM. Basically, while one stick of RAM is storing/retrieving data, the other stick is transferring it to or from another part of the computer and vice versa. This back and forth saves a whole lot of total transfer time.
To use it, all you have to do is make sure two sticks of RAM are in the appropriate channel slots. So you might want to start with two 512MB sticks in the channel A slots, then maybe upgrade with another two 512MB sticks in the future for the channel B slots.