I second the Mac Mini, however buy a late 2009 Mini with DC supply. Sounds better than the AC versions. Instructions here:
http://www.empiricalaudio.com/computer-audio/
http://www.empiricalaudio.com/computer-audio/recommended-systems
Steve N.
Empirical Audio
http://www.empiricalaudio.com/computer-audio/
http://www.empiricalaudio.com/computer-audio/recommended-systems
Steve N.
Empirical Audio