nehalem processors have an integrated ddr3 memory controller which supports 3 memory channels.
some information regarding specing out a dell poweredge r710…
the server has 18 dimm slots, all 18 slots may be used when filling them with single or dual ranked dimms (up to 8GB single dimms). only 12 dimm slots may be used when filling them with quad ranked dimms (16GB).
the dimm slots are divided 9 by 9 for each processor socket. 9 / 3 equals 3 dimms per channel.
if the channels are going to be fully populated one should buy 800MHz dimms, because that’s the speed they’ll run at in a full channel configuration. 1333MHz dimms can only operate at full speed in a 1 dimm per channel configuration.

info regarding nehalem: http://www.anandtech.com/cpuchipsets/Intel/showdoc.aspx?i=3448&p=5