Questions About Dual-Channel Memory

My laptop has 6gb of RAM. One 4gb stick and another 2gb stick. I've heard that in order to utilize "dual-channel" memory, both sticks have to be the same amount od memory. Is that true? I was thinking of ditching the 2gb stick for another 4gb to get a total of 8gb of RAM. I'm not necessarily considering it because I need more memory, but rather to get dual-channel support as I have heard that it provides a performance boost. What do you think I should do? Is there anything I should know before going out and buying another stick?