the advantage is:
with two identical sticks of ram the motherboard activates dual channel , if they are not identical the system can not adjust for voltage and other differences that are caused by ram of different specs.
lets say you have 2 kingston 512 pc3200ddr. you put both on the blue slots. you have dual channel. if you are going to add more, you will have to add another matched pair(does not have to be the same as the ones on the blue strip) to the black. if you add one strip to a black slot you will detect and use the ram in single channel mode.
what the big deal is with dual channel is you have two pipelines to the processor from the ram instead of one.
the white paper explains this better then i can.
http://www.kingston.com/newtech/MKF_...Whitepaper.pdf