If it isn't even going to be used for playing back files locally, you don't even need a pentium 4/athlon level CPU for the job (though you'll have trouble finding anything with SATA or lots of IDE ports with a P3 or something
)... Just figure out what CPU you can get for a motherboard with the drive connectors you need, that draws the minimum amount of power. No point in putting in a power-hungry CPU (I'm looking at you Athlon/Sempron and Pentium/Celeron) to just act as a file server; might as well minimize the electric requirements, for the lowest cost of running it. Look into micro-atx style motherboards/processors; some sort of Pentium M CPU would be great. You can still use such systems for local playback if you need to, naturally.