it prolly doesnt make a diff right now. 2 sets of duel(im guessing) would perform the same as 1 set of duel... only different would prolly be if u actually use up 2x512mb ram... then it would beable to use then next set in duel mode if need be... but i dont think most ppl use up that much ram at pressent time.
the only real diff i imajine would be if and when thy come out with quad channel DDR... that would help