My understanding:
Old style (Single data rate SDR) ram - accessed once per cycle. Cannot be run as Dual Channel.
New style (Dual data rate DDR) ram - accessed twice per cycle. Can be run as Dual Channel.
Single channel access is done 64 bits at a time from (example) memory location 0 through to the end (depends on how much you have, so let's say, with 4 x 512 you have 2048, so 0-2047) so the memory manager grabs 64 bits, moves and writes it, goes back. etc ( starting with Bank 0 and moving the the last bank (bank 3 in a 4 bank system )).
IF the motherboard uses Dual Channel controllers for the memory management,and the RAM is installed properly in both A & B banks, then the DDR memory is accessed at 2 x 64 bits = 128 bits at a time and written 64 bits to one bank of ram and 64 bits to the other for an actual transfer of 128 bits, which saves some time from digging out 64 bits, writing it, another 64, etc. This process is approximately 15-20% faster than 64 bit single channel access and writing.
BUT - the motherboard MUST support Dual Channel as well as Double Data Rate.