Okay, I know people have been preaching this best friend thing, lately, but trust me sweetie: In the long-run, it doesn't work.
Now, let me elaborate: Trying it will not harm you as much a it would if she were a stranger. So, since you're best friends, make it known to her and have the short-term pleasure, but don't expect any long-term effect.
The good thing about going out with a
best/really close friend is, when and if you split (which is most likely to occur) , you'll still be able to remain friends/revert to friendship. Contrary to that, a relationship formed by other means are usually hostile/irreversible once things go wrong.
So, don't be afraid to ask her out, but don't expect a long-term effect either. Additionally, if she says 'no' or feels uncomfortable about it, it creates some tension between the two of you and can cause some rocky periods. My sis was and still is in such situation with her supposed best friend. They are still "best friends" though!
