dual channel ram means there are two ram pipelines to the cpu to try and help the bottleneck caused by the system bus. on all the dual channel boards i have seen if you have one stick of ram it runs in single channel mode. to get dual channel to work you have to have a matched pair of what ever ram you intend to use, like if you bought 2 sticks of 256 or 512, make sure they are the same brand and spec. install both in the same color slot, like if you put one in the blue slot, put the other in the blue slot. this will make the dual channel activate. if you mix it(one black, the other blue), it will still run just not in dual channel mode. if you decide to add more ram you will need another identical pair of ram to go in the two open slots, if you only add one more, it will see the ram but your back to single channel mode. it is very important to have the exact same chip in both slots of the same color.
RAM tranfsers data in "strobes". Just like a strobe light, it cycles on and off. Well, what dual channel does is offset each pipe so that one stick is strobing on, while the other is off.
follow the link for more info on dual channel ram its a .pdf file so u will need adobe