For motherboards that support dual channels, 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.
You need to 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 blue, the other green), it will just not be 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 going to be back to single channel mode.