What's better, dual channel or single module?

Dual channel kits are tested and guaranteed by the distributor to work together in dual channel mode. A distributor may also sell the same RAM as single DIMMs untested together.

Download CPUZ and install and click of the SPD tab. Memory tab will state dual or single channel function.

http://www.cpuid.com/cpuz.php

So it's possible that both types of modules can work together, but not positively? What is the performance difference in double-sided