Many users have told us that they have met problems with their Mac Pro rev 2 and the memory. Up to now there have not been this type of problem. It is simply that the order of installation of the memory cards has changed for the Mac Pro rev 2, and if you do not respect these rules you will have problems concerning the recognition of memory. Thus here, pulled from the documentation of these machines is the order to follow: If you have only two memory cards, you must put one in the top riser and one in the bottom. as with the ancient Mac Pro. Beyond this, things change. It is necessary to put your pairs of cards side by side, and not distribute them on the two connectors. Thus, another more visible example, if you have 3 pairs of cards, you will put two of them 2 in the top and only one in bottom. [translation by crispin]

	Read more at Mac Bidouille
	www.hardmac.com/news/2008-03-15/#7951