Are you sure you're not all of the sudden into her just because she now has a new boyfriend?
Make sure you would truly want this girl as a girlfriend before 'flirting' any further. Regardless, let it be up to her wether she dumps the new boyfriend or not. Do not try to break them up.
the fact that she asked you what she should wear means she was fishing for a compliment from you. she obviously was "testing the waters" to see if you were interested in her. it sounds to me like she's not too serious about this new boyfriend if she's giving you so much attention.
just be honest with her - ask her how serious this new relationship is and if she'd consider going out with you sometime. if she says she's not interested, i don't see why that would harm the friendship.
I'd say, if she has a boyfriend now, lay off her for a little. When they break up and she is looking for someone to turn to, make sure it's you!
I'm kinda in the same postion as you, but my "girl" friend is the biggest flirt in the world, so i can't tell if she likes me of if she's just flirting. Being just friends with a girl is hard, because there will always be an attraction....
this is why when i break up with a girl for reasons other than we don't like eachother, i tell them straight up "i can't be friends with you." Once I've had feelings for someone, even if its before we go out, I can never be just friends with them again
kidna hard sometimes, but a lot easier in the long run cause you move on quicker