Hopefully your real name is not as lame as your posting name.

It appears that she is scouting the field, she isn't 100% on the present BF and looking around. When somebody is madly in love, logics do not matter so whatever I tell you, you will go and do what your heart says...i.e. meet this girl. But logically I wouldn't do it.

First of all she has a bf, official or not. That's too much headache to manage as she is bound to compare you two and bring up stuffs. Second, let's say she ditches him for you, given her history she might continue to scout the field and "cheat" behind your back to find other better suitor. She will never be satisfied. Note that cheat is in quotes, this is rather emotional cheating.

Follow your heart and do what it says. If it breaks, tell it that it is its own fault. If you still have control over yourself, walk out.