I will admit outright I also only really know gaming/editing machines, this isnt my strongpoint.
I know that when it comes to NAS (network Array Storage) machines the CPU doesnt have to be all that powerful. A much bigger emphasis is placed on the Network Interface Card, which will have to be somewhat strong to stream movies to multiple machines.
Have a look at the NAS build thread at the top, those are what you should be looking at.
This may help as well.http://www.youtube.com/watch?v=ycbq_gTqT5M