Don't even bother, IMO. If she's got another guy on her plate, you're just setting yourself up for hurt. It's not worth the potential happiness you could have with her.

Also, take a step back and make sure you're not just caught up in the moment. Do you really like her as much as you think you do? Just try to give yourself a tiny bit of a reality check to make sure. If so, see what the situation is with this other dude, and perhaps persue a relationship with her.