If it was me, I would tell her I had some feelings for her and ask her out on a date. If you like her, why not? if she loves her boyfriend, she can just tell you that - you wont really have lost anything , and you could always claim that you thought they werent going out. She doesnt "belong" to this guy anyway, if she wants to be with you there is no Karmic reason she shouldnt as far as I can see.
It sounds to me like you like her a lot, and if you dont go for it, you will probably regret it and be thinking about what might of been, so if I was you I would just go for it!
