The irony is that you have to ask us here, i mean the Escapist is probably not exactly known for good Relationship Advice, given that we're kinda all a bunch of nerds and geeks here but well lets try.
That you already ask us means that you are considering sleeping with the new Girl. Bad Idea. She told you point-blank that she does want to sleep with you, that implies she wants if anything only a physical thing from you, at worst its a setup for a bunch of other, not very fun things to experience.
So my advice is, tell said new Girl you are taken, then tell your real Girlfriend about it, do not mention yet that you told her you had a GF. Simply tell her that some Chick made advances on you, wait to see her first reaction. Depending on how she reacts to the news, go from there. If she inquires further into it without getting "angry" at you or otherwise go nuts, tell her that the Girl was making advances, you replied by a bit of innocent flirting and when she wanted to go further, that you told her off.
If your Girlfriend goes mad at just the news that some Girl asked you for a one night stand, call it off with her as well. Because you arent going to have fun with her either in the long run.