The Digital Audio Jukebox is based upon a PC running Linux - I prefer Debian, but you could use whatever distro you like.

CPU - The lowest spec machine I have built is a Pentium 75 CPU overclocked to 110MHz, which allows me to playback 2 stereo tracks simulaneously, with around 20-25% CPU usage per zone. My current server has an AMD Duron 1200 CPU, which uses approximatly 1.5% CPU per output zone.

Storage - You'll need a hard drive to hold the operating system, Digital Audio Jukebox server software and if you wish, the MP3s. In my Jukebox I have only a small HD to hold the OS and software, and I mount a partition of MP3s from my fileserver using NFS - but you could keep the music on a local HD if you like. 

Control interface  - You will probably want a network interface card in the Digital Audio Jukebox and then use another PC or network device to control the transport functions (PLAY, PAUSE, STOP etc), build playlists, and download tracks to the unit. But you can run the client locally instead if you want, maybe in conjunction with the lirc client. Client/Server communication is done through TCP/IP sockets.

Audio output - A sound-card is required to interface the Digital Audio Jukebox to your amplifier(s). The server supports multiple soundcards for simultaneous decoding of multiple songs to create 'zones' - this is to provide whole-house audio as part of a home automation system. You can use any soundcard that is supported in Linux, however I prefer to use Soundblaster Live 5.1 cards in my server. The advantages of the SB Live 5.1 cards are:
    You get 2 zones per card, by configuring front and rear outputs as separate dsp devices (using Creative's emu-dspmgr utility and this script,
    You get an SPDIF output.
    They are relatively cheap, easy to get hold of, and sound quality is not bad.

I would recommend avoiding the cheap 'no-name' sound cards, as often the sound-quality is bad (high levels of 'hiss'), and you may find that although the card has an SPDIF socket, it's not actually supported by the chipset on the card!

A typical system configuration might look like this:

I have my server and amplifiers in a central location, and run the audio at speaker level to each room, except for the living room where I run SPDIF to my AV amp. I use a pair of 4 channel car amplifiers, which provides stereo audio to 4 rooms. Here are some pictures of my system:

	Top left is the main HA server, running Misterhouse, NFS, Samba. Bottom left is the Digital Audio Jukebox server. Bottom right is the firewall/NAT router (Freesco). The monitor is a Compaq 12" monochrome, rescued from a skip!

Home made patch panel in the centre, made from a 19" rack vent plate. I have mounted 4-pole Speakon sockets so I can patch the audio on a room-by-room basis.		The two, 4 channel car amplifiers bolted to a 19" rack shelf.
Serial IR interface module, for lirc.		3 Soundblaster Live! 5.1 cards. This gives 6 separate stereo outputs. Top left output goes to the AV amp, top right goes to the wireless headphones transmitter, and the remaining 4 go to the car amplifiers.