Memory controllers are 64bit (usually). A dual channel system has two of them running concurrently rather than one. You *theoretically* get twice the bandwidth, but few systems actually use all that is available. Pentium 4 Northwood and later benefit the most from dual channel, s462 the least (so far). S939 usually gets about 5% better performance from dual channel (comparing s754 to s939 with CPUs running at the same frequency).