I have read about Dual Channel only giving an extra 1% to 5%. Either way, its pretty small.
A lot of people get DDR confused with Dual Channel. Nubius gave a nice explanation on Dual Channel. DDR just means that the RAM transfers data twice for every clock cycle. For example, if a stick of RAM is getting a clock pulse that is going at 200MHz, it will transfer data 400 times per second. This is how DDR RAM receives a frequency of 200MHz, but then it is called DDR400.