Since you asked here's my take. Dump the girlfriend first then go for it with the babe. Unless you really think your current girlfriend is the one you want to spend the rest of your life with then go experiment. Even if you are sure think about it really hard. You are 22! Go take some time to have fun and grow up before you lock yourself into that kind of commitment. Don't get me wrong. I love my wife very much but we dated a long time before we got married this past July, and I am almost 30.
What I don't think you should do is cheat on your current girlfriend, that is too weasely. Dump her before you get your groove on. That at least leaves the door open to getting back with her later if the babe doesn't work out or you decide that she really is the one for you. You are almost sure to get caught if you cheat on her and that is pretty much a guaranteed way to make her hate you the rest of your life. And if she doesn't hate you for it then she is too stupid to marry.