Ah, a chat shorthand addict...
If (on the main page, right above the threads) you enter "copyright" into the supersearch, you'll recieve more answers than you'd ever possibly have any use for. It's a fairly common topic, and most discussion centers around either UK or USA applications.
In the meantime - the acts you name use a LOT of traditional material. An easy way to tell if it's going to be an issue is to see where they've attributed the song. If the arrangement is copyright protected while the lyrics themselves are traditional, as long as you aren't using their arrangement you're safe. And as to the "50 years or older", to be ABSOLUTELY safe you're better off just calling it at 100 years. Given the influence of the corporate sponsors over the process, in the US it's up to 70 or so years, and chances are it'll go even higher - but if the author's been buried a century, I don't think anyone can legitimately argue that the work isn't public domain.