do yourself a favor and don't freak out over a girl before you know what she's like in a relationship
right now you think talking to her will be disastrous and you'll get rejected but is that even so bad?
don't waste your time chasing her if she's never going to be interested, so i'd say talk to her as soon as you find out she's single (don't try to mess around with girls with boyfriends for your own sanity, it's really a lose-lose situation because she's probably not going to leave her boyfriend for you and if she does you know that the same thing could happen where she leaves you for someone else when you start to bore her)
i've only just realized it isn't the end of the world if a girl isn't into you because you really do control how the world is around you, if someone doesn't want to be with you then you shouldn't want to be with them and that's pretty much the way to handle relationships. not to say it has to be perfect but being a typical nice guy and waiting around for her to come onto you first is only going to work if you're really attractive.