If you are so sure about it, then I would advice you to pursue what your heart desires. Seems like you have put sometime for preparation and thought about what you are going to study for the next 3-4 years, if you are REALLY confident, then my advice is JUST GO FOR IT!
I am no expert in this, so take this with a grain of salt. As far as I know (and heard from people around me) a letter saying - "Mr. X got an A in my Topology class" - won't help you with admissions that much, no matter who wrote it.
True, recommendation letters from popular people will sound...
I don't know if ETS has released the tests or not, but I am sure they are not available for free, not legally atleast. But I found this website useful - http://www.hcs.harvard.edu/~physics/?q=node/13
At the bottom of the page there are some notes and practice tests.