I agree with both points above, but I'd make a separate unit for a jukebox, you'll be glad you did. The only issue with the juke on an arcade machine is, you would have to close the jukebox, to play the arcade or vice versa. You can run them both at the same time, but the sound is terrible and you would have to get out the mouse, reduce one to add/play the other. kinda takes away from the arcade experience! several really good ideas in the mp3/jukebox portion of the forum, some are simplistic and some are a little elaborate. Just what suits your wants/needs by budget and style!
Go Ahead, build that jukebox! You already get the 'WOW' factor with the arcade cab, get the 'WWWWooooWWWW' factor with a dedicated jukebox as well. All you really need is a P3, 512mb memory and a cheap touchscreen from ebay to set one up!
If you build one, take plenty of pics for us all to see!

Fordman