i must say averett, i was surprised to not see your staple "you're taken, don't even consider it" response.
that being said, i agree with what everyone else said... you need to do some self-assessment and figure out what you're really looking for at this point. once you decide, do the right thing and support your decision... either by breaking up with your girlfriend and pursuing the fling, or by turning down the fling and staying true to your girlfriend. don't get greedy and think you can have both... it rarely works out.
good luck.