There are energy efficient cpus you can use. AMD have 65 watt versions but there are small for factor things that run on a via chip or an Atom system running @ 30 watts with a raid controller would imo make a solution
http://www.mini-itx.com/store/?c=47#d945gclf example of an atom
actually these atoms look pretty cool. 35 watts out put under load to a tv obviously add your hdds you have a solution for a nas at very little cost