Dual channel is dependent of the mb
sorry, that isn't entirely true. dual channel is dependant of the memory controller
. on Athlon 64's and Opteron's that is integrated into the processor. of course part of the compatability relies on the motherboard but for the most part it is the memory controller.
now the guy at the store is an idiot, clearly your motherboard is s939. it doesn't take much to find it is indeed s939 from the info you provided
history time. Socket A first supported dual channel, though only with the Athlon XP or later. that depended on the motherboard entirely as at this point the memory controller was still on the motherboard. s754 had the memory controller built into the processor however the "IMC" (Integrated Memory Controller) was still a work in progress at that point and could only support a maximum of 3 sticks of memory and no dual channel. s939 supports dual channel.
edit: bluntness of post removed, sorry RumbleNJungle