It's great that your friend is cool with the fact that you like a girl, but the true test is how she will react when she finds out she is the girl! There are so many things that could influence her reaction. Some of it may have to do with how open she is to such relationships and whether she is curious about other girls (or you in particular).

Her reaction may, in part, depend on how you handle the situation. You might try checking in with yourself first. How much does this mean to you--enough to test the friendship? Are you prepared to let it drop and find a way to deal with your feelings if she tells you that she is not interested in you in the same way? What if she IS open to it? Will you be too enthusiastic to allow her time to catch up with you? Remember, you've probably been thinking about this a lot longer than she has.

If you do decide to test the waters, try not to pressure her. Give her time to think about what she wants, but take her word if she tells you "No, thanks." You never know, though, she might just say, "Me, too."

Good luck!