I built one from spare PC parts I had. Added 700GB of hardisk storage and hooked it up my home network(hardwired). It's serving all my music(MP3 and FLAC), my movies(*.vob, MPG1/2, .mov) and my pictures( JPG's ) throughout my home. It's idling when not serving so I plan to run some video converters in the background mode and maybe P2P.
It's doing what I need to do - serve media files to bedroom, living room and nook. I highly recommend it.
FYI, it's AMD XP2100, 512MB memory, no CD, no video card(integrated video), no sound card.