Break up will current girl straight away - whether you're going to see someone else or not - cos neither of you should be in that relationship.
If you really want to get it on with her friend, wait a while til you and 1st girl are ok again and you can talk to her sensibly and ask if its ok for you to see her
Break up with your current girl. It does not sound like you two are compatiable.
You are not committed, go for it
I think what all of the lovely ladies above have said is absolutely right.
If you feel this way about your current girlfriend, then why would you wish to continue?
Do the right thing, finish with her while it should still be possible to have an amicable split.
With regard to her friend, I would definitely give it some time before approaching her. Often times, when people are already in a relationship, they can seem safer and more attractive than when they are single.
Once you've split from her friend, see if she's still interested, see if you're still interested...
Tread carefully, this situation has complete FUBAR potential...
(good luck)