Ganged mode basically allows all the cores of the processor to see the whole lot of memory as one big block while unganged mode gives an equal but separate portion of memory to each core to play with.
Unganged is better for multitasking as each core has its own dedicated memory portion.
Ganged is better for games or other programs that will take a massive amount of memory while running as all cores can get easier access to the big memory file since the memory isn't portioned out.
Its up to you really, i have it ganged and sacrifice a tiny bit of boot up speed after log on to make sure my games are running optimally.
i think you can change that setting from the bios
however, as i've read online, it wouldn't make a whole lot of a difference
also give these two links a look, they might provide you with some useful information