I believe that you don't choose a religion, a religion chooses you.
Just started going to Universalist Unitarian myself. They seem to be Very Nice People. If anything, they aren't religious enough for me. No problem, I'll get my religiony stuff from having semi-paganish thoughts, and my human stuff from the UU's.
If your GF simply says "a Christian" without a denomiation (protestant, methodist, lutheran), then she's probably fundamentalist and they don't seem to compromise well.
It may help you to go to Beliefnet.com and take their quiz: Belief-O-Matic - Beliefnet.com
It's a website commited to help people to choosing a faith (or having a faith choose them?).
5'0" female, 45 years old. Started Primal October 31, 2011, at a skinny fat 111.5 lbs. Low weight: 99.5 lb on a fast. Gained back to 115(!) on SAD chocolate, potato chips, and stress. Currently 111.