I recently picked up a used netbook exclusively for audio use.
I wanted 2GB RAM, a reasonably fast processor, and a 250GB drive and gigabit ethernet. I actually store my 1.5TB of FLAC on a USB drive, but wanted a fairly large drive in the event I changed my mind.
I deleted all MS off it that I could, loaded up Logitech Media Server, and I run it wired.
Was less than $180 off Kijiji.