Here's what I know...
(1) Most women don't let their husbands be BFFs with another girl , so if you are afraid to cross the line because the two of you are soooo close, get over it, you won't be that close forever.
(2) You might be surprised that your friend has those same feelings for you.
(3) The best relationships start as friendships, so if you are that lucky to have a friend in your life that you are romantically attracted to, go for it!
(4) If you go for it, make sure you do it over a bottle of wine, it will be easier and it will give you a built-in excuse, if its a disaster!
(5) If you go for it, keep the talking about it to a minimum; talking with just scare you both off
(6) If you do talk about it, keep it relaxed and make sure you that you are reassuring to your friend that you can handle it, whatever the outcome, so that your friend will not resist out of fear of losing a friend.