[Adam the Fish]

Haaang on, let's not be overly generous to Theists.

Yes, they 'prey' on the vulnerable (hooray for fulfilling the "Militant Atheist" stereotype). Now, I'm sorry, but I find it hard to believe that religion could exist without authority figures telling the impressionable about it. This is mostly in young children but also other impressionable personalities.

My atheism aside, the others have given excellent advice. I don't think you need to worry about changes in your relationship too much (as much as I'd love to paint the opposite picture, the people your girlfriend is now worshipping with are likely very kind people devoid of mad extremist views). However, I'm sure that she'd be more than happy to have a Smiling Assassin Jesus if you offered.

If she starts making you listen to things against your will or otherwise forcing any religious views upon you, it is the simple matter of telling her that, as you did for her, she should do for you. You don't listen to Atheist music ("Gravity, Gravity, Gravity" and "The Genetic Mutations occurred Two by Two, Evolve!" etc. ) around her, she should respect your views.

Religion might affect personality, but the personality is still there beneath religion and I'm sure it won't cause problems between you.
I'm strongly Atheist, but I still think that tolerance and understanding are the best things, not just all agreeing.