Oh, one thing I forgot to mention. When I came out to my parents, I wasn't sure what their reactions would be, so I asked to meet them for supper at a favorite (and always busy) local restaurant, so we could talk. By doing that, I controlled the situation by making sure they couldn't explode or do anything truly negative, action wise, to me. I figured by telling them in a public place where you could still maintain anonymity, that I could temper what could be horrible reactions, while not stifling good reactions to much.
So, a date at a favorite restaurant (but make it a more casual, louder one, nothing too quiet) may be the best place to talk with your girlfriend. Just make sure you tell her in a way that opens it up for questions, be prepared to answer anything and everything as best you can.
Just my additional 2 cents, I hope it helps.