dude, just hang out with the new girl and see what happens. if you start throwing down with her, then you should consider breaking it off with the current girlfriend, but if you are just hanging out with the new girl, and not doing anything more than maybe giving her a smooch here or there, it's no big deal, and it's not "cheating". if you were married then it would be completely different, but you aren't, so eff it.
i mean, don't get me wrong, the whole situation might end up giving you a massive headache, but at the same time, sometimes hanging out with a new girl can be a good thing. it can remind of you why you like your current girlfriend in the first place, and show you why you're with her and not some other broad.