The dual channel mode was introduced when the DDR memory came about.  This technology requires two matching pairs of memory modules to be installed so that that the system runs at optimal performance.  In dual channel mode, the effective bandwidth becomes twice the standard size, that is, if you have a Dual Channel PC3200, the peak bandwidth becomes 6.4GB/sec instead of 3.2GB/sec.  This is because the two sets of identical memory doubles the data paths that can be used simultaneously allowing data to move faster, minimizing time delays between read/write processes.  Before you upgrade, consult your system's specifications to verify if your motherboard supports the dual channel function.