yes it means it is better two have 2x256mb than 1x512.
let's say you have pc3200 memory, instead of having a transfer of 3200mb/s(3.2gb/s) you will have a transfer of 6.4gb/s.
this is due to the 128-bit controller. in a time span, twice as much data can be transfered than with the normal 64-bit controller, which is "normal" ddr.
it is twice as fast !