Shaddowval has a great perspective, and some good advice. ^^
From my perspective, as a girl, I see your girlfriend as being a real friend to this other guy, and most probably not in a romantic way. I say this because I know that I always feel more secure and flirty around friends that I feel no romantic attachment for. And yes, almost every girl I know will flirt a little just to see what response we get out of you (as a guy, to see if you care). However, we do this naturally and do not mean anything by it, so if you accuse us of becoming "too friendly" with another guy, of course it would make us cry. =\
We want you to be proud of us, and we want you to be our man. If we flirt with someone else don't sit on the sidelines, confront us in n indirect way by pulling us aside and flirting with us.
We naturally feel less secure around the one we have real feelings for, and obviously you do as well, but as a girl we will generally not take the lead in the relationship (depending on our personality and life experience) and so you will have to overcome your insecurity and also overcome your jealousy (despite the fact that we will likely make you jealous very often). It isn't 100% on purpose, as it is a natural thing to do that we really don't give much thought to, but your reaction to that affects us greatly. You have to make that insecurity go away by making us feel comfortable and secure around you (in public and in private) so making any form of accusation, true or not, should be saved for the times when it is really important. I know that as a girl I will take a guy in every argument without thinking, it's my nature to do that, however guys generally are very good at "choosing battles" and only fighting those that really matter (to both partners in a relationship).
So, basically what I'm saying is, don't read too much into the flirting with the friend and don't lash out based on that jealousy/insecurity as it would definitely turn me away. If your girlfriend does love you, then she will forgive you for most of the things you say, but she will never forget it - so it's best not to create scenarios that you wouldn't want her remembering. =]
One last note: don't be so clingy to her. This situation sounds to me as if you are being too clingy. There is a line where "close" becomes "overbearing". Don't cross that line or you will lose her.
I hope that helps and makes sense, and I know my perspective isn't the best and possibly doesn't completely apply to her personality or yours, but hopefully it was somewhat helpful.