Also, you should know that all of the best students in the world will be applying to top15 schools. A lot of these students will have had the opportunity to work with or study under celebrity professors and a lot of them will have professors who are much more well known than yours. So don't think you are in an exceptionally good situation there.
Also, as jonthawk said, it is not so important who writes the letter as what is in the letter. Of course, it always helps to have professors who people know, but that comes second to the quality of the letter.