I have to admit this just does not sound that great to me for several reasons.
First, why go after someone with a boyfriend? I mean if she is with him but flirting with you... well why would you want her? If she'll go behind his back she'll go behind yours. If she won't respect her commitment to him, she won't respect a commitment with you.
Plus, you know she's flirty, so if you do get together you'll be paranoid all the time that she's going to be flirty with someone else.
And what does it say about your integrity that you would break up a couple? This would probably hurt him and you and the girl would come out of the breakup looking pretty bad.
On top of that, who has the energy for all the heads trips you've been going through over this?
I say wait until she's out of the relationship, wait a respectable period of time, and if you still have the same feelings for her, invite her out to sit down with you and have an honest conversation about your feelings and see if she's on the same page... THEN move forward.
It seems like you've put the middle and the end before the beginning. That's not the way happy stories go.
