i'd wait but if it dosen't look like they'll be breaking up anytime soon then move on cause i know i wouldn't be single for ever waiting 4 some one 2 break up with her then she has to get over it so if u really think she's the only one wait.
a girl with a boyfriend....man they all have boyfriends....no reason why you can't both move on AND wait and see. Maybe while your waiting and seeing you'll meet someone that makes you forget all about the other one.