Don't be so quick to jump into a relationship with your friend. This girl was my best friend during college and then her and I moved into an apartment together. I think I pushed the issue because I wanted her so bad, and I think she more or less just decided to go along with it.
I had been going with this girl for over 2 years when I found out she was cheating on me for 1 of those years. Revenge and anger were my first thoughts. I started following her and I hired a private investigater, I was renting movies like Fatal Attraction , I was headed towards stalker-ville.
But I got some help and I'm finally over it. Maybe that was all a bit tangental, but my point is, if you're not sure then don't rush it. It could cause you and her a lot of extra stress and it's better to just take your time and see how the relationship develops.