From a designers POV background music is not something that should be encouraged. Firstly not everyone browses in silence. If you were listening to your favourite track and some dodgy site started playing Spice Girsls at you you wouldn't be best pleased. Secondly for someone on dialup you don't want to slow down browsing with all the extra data. Thirdly it is generally considered Naff.
If however you want to ignore all my advice then
http://www.thesitewizard.com/webdesi...undmusic.shtml
Pretty much covers it