Its not a case of the options switching you from Dual to Single channel at all.
Its is an option that affects how the memory is addressed by the CPU.
When running ganged mode your memory controller will operate in 128-bit mode. This has the benefit of giving the whole CPU a massive memory bandwidth. When handling one application that greatly benefits from higher memory performance this is the option you want to be sticking with.
Unganged basically will split that 128-bit memory bus into 2 x 64-bit, so you will essentially have two memory buses. Because of this, Unganged is particularly useful If you are a heavy multitasker, as you are packing 2 memory controllers effectively.
It will not force single or dual channel setups. Your memory will always be dual channel regardless of which configuration you choose.
It is simply a case of how you wish your system to be tuned. If you are using your PC for a live audio take, handling multiple tracks, then ganged will benefit you with the higher memory bandwidth.
If you are wanting to do some encoding of some movies while you have a quick blast on CS:S, however, go with unganged, as then one half of the memory controller will be dedicated to each running app.
