I don't have a "need" for a server, BUT if you are wanting one that will not use gobs of power.. Perhaps consider starting with a
MINI ITX ATOM system board.
This board is in the Mini ITX form factor so it is extremely small, has built in Dual Core 1.86 mhz Atom low power processor. Extremely low power draw at about 16W for the whole board including the processor and the heatsink does not require a fan..
This board and processor would make a great "server" but it does have a few minor issues. If I remember correctly it has one PCI and one internal MINI PCIe slot and two SATA ports.
I used a slightly older 1.6 mhz Atom board a few years back for my "camping" PC and it works great for that function. The only major drawback to the Intel board is the weak on board graphics is not robust enough for AVCHD High Def video. Other wise works perfect for my needs..
If you look around there is other MB manufacturers which also use the Atom processor but they use better on board graphics hardware which make them perfect for media players...