[ctviggen]

The only thing that concerns me about a fanless computer is the extra heat killing hard drives.

Put the hard drive on an eSata connection and in a box by itself.  I have a fanless computer with no moving parts. Of course, I use my 3TB drive only for back up and to record movies, but I don't see why music wouldn't work well. 

[WGH]

The AMD Phenom II X2 565 features "energy efficiency innovations include Cool’n’Quiet and AMD CoolCore, which reduce power consumption by balancing the processor activity, letting you enjoy environmentally friendly, energy-efficient performance."

The Biostar A770E motherboard supports AMD Cool'n'Quiet Technology.

The Antec Earthwatt's "standby and no-load performance were both reasonably efficient, with standby coming in well under one watt, and no-load at a little under ten." [Silent PC Review]

I think most of the time the computer would be cruising along close to no-load with both the processor and power supply using very little power. Based on the Calculate your electric cost online calculator, using 10 watts 24/7 at 15 cents per kWh the server would cost $13.32 a year to run.

[Markhh2]

I was pretty happy with a vortexbox feeding my touch, but I picked a dell zino home theater pc for less than $400 and and very happy with it's size and performance.