AFAIK you need two sticks to run dual channel that are of the same size, otherwise you run single channel. I am running 3GB in dual channel and the configuration that i need to do that is 2x1GB and 2x512MB for a total of 4 slots used, the 1GB sticks in channel A and the 512MB sticks in channel B. (on an ASUS A8N-SLI Deluxe)
the debate can be made about more RAM in single channel or less RAM in dual channel, I am not an expert so I can't give expert advice, HOWEVER, my experience and my perception is that it is better, if we are talking GB's of RAM to have 3 v 2GB in single channel than 2GB in dual channel. again, that has been my perception, im sure there are benchmarks that can prove or disprove my perceptions. if we were talking 2.25GB in single channel and 2GB in dual channel, i would go with the 2GB in dual channel.