mccloud said friendship to relationship rarely works, but i dont know if thats quite right. If you are going to ask her, know that if she refuses you, you cannot make it awkward if you want to keep your friendship. Just accept the fact that she doesn't return those feelings, man up (aka don't let them see you cry,) and continue to be a supportive friend. Don't act like it never happened, and give her some space if she wants some for a bit. If it works, great, but if it doesn't, listen to Livvers and stay friendly.
If it works for you, then thats great. Make sure to keep your friendship visible throughout the relationship. I had a great friend for about a year in 8th grade, and then we dated solidly from 9th to 12th. So it works, but you need to be careful about it. ;D
I believe KDJ talked earlier about the friendship --> relationship and relationship --> friendship transitions. it would be worth your time to skim some pages back and find what he said.