Stick with two memory sticks rather than one as the motherboard supports dual channel, though the actual benefit may not be noticeable.
Just because you can, doesn't mean you should or must.
At 2x2gb, dual channel is going to be a largely theoretical endeavour, and also is a waste of slots and will be a pain when it comes to putting in more ram because he'll probably end up having to remove and throw away those two chips.