the easy route would be get the hard part done for you :)
http://cgi.ebay.co.uk...H&hash=item1c16e49940
If not, and it's just music then you don't need anything special, an atom based computer would be enough, one with a digital audio output would be best as long as you have or get a suitable amp.
Windows media centre works well with a touch screen, as does the squeezebox player or if you look on google there are a few linux based projects.