While the 64bit vs 128bit is accurate, dual channel (the 128 bit) is different from 'dual' data rate mentioned above - DDR is DOUBLE data rate so if the true freq of a stick is 800 then as DOUBLE data rate you take true freq 800 x2 = 1600 effective. It runs at this data rate regardless if it is in single, dual, tri or quad channel mode, In dual channel mode instead of each stick being seen as an individual 64 bit device, the MC (memory controller sees all the DRAM as a single 128 bit device, which would theorectically double the bandwidth, but in actual practice will provide upwards of a 10-15% performance gain