As Soulwynd indicated, you need to take a look at your own thoughts/feelings before worrying about her's.
What, exactly, do you mean when you say you're crushing on her? What is it that you think you're pursuing? What is it that you want, that you don't have in your current relationship? What is it that you want to attain?
How would you feel if you continued to be "just friends", and she had a romantic/intimate relationship with someone else? What would that do for your friendship?
How would you feel if she rejected you? If she's happy with the relationship as it is?
To be honest, you're more or less doing it the right way. You've actually gotten to know her before trying to get in her pants. You'd be amazed at how few people do that these days. And people are still surprised when they see the divorce statistics...
You can't control her actions, emotions, or response. She's either interested, or she isn't. She'll either get weird after you broach the subject, or she won't. And that's entirely out of your hands.
But you can control your own actions, emotions, and response. That's what you need to worry about. Evaluate what you're feeling, what you're looking for, what kind of things you can tolerate, and what things you cannot. Then make the appropriate decision.
Although... To be completely honest, it almost doesn't matter. You said you'd known her for 10 years, since you were both 6? The odds of the two of you actually staying close friends through the rest of highschool/college/life/whatever are pretty slim. It'd be a shame to pass up the opportunity just because you thought you'd lose a friend, only to lose her anyway.