Ok, so i bought a mac pro with 1gb (2x512) now, I would eventually like to upgrade to more and was wondering if I buy say 2gb (2x1gb) would It matter if i put it on the top riser or bottom since they are different sizes....Would it be better to buy 2x512 and have that on the top and bottom and then buy larger ram sticks for the rest of the slots....? Anyone know? Or does it not matter mixing in different sizes of ram sticks?