First, CAMPEP. CAMPEP accreditation is given to schools that apply and meet a certain set of criteria. You can read about the criteria online but the important thing is that it makes it easier to go into clinical work. This is because after 2012 graduation from a CAMPEP approved graduate program or residency will be required to take the ABR exams. Passing the ABR exams are not required for participation in clinical work but physicists who have passed them generally command higher salaries.
There are relatively few CAMPEP accredited schools in the US. The list of accredited US schools is here: http://www.campep.org/campeplstgrad.asp
The top schools for medical physics are usually taken to be (in no particular order):
Duke, Wisconsin, Chicago, UT-Houston and UT-San Antonio. Columbia was just accredited so I don't know where they fall, although I'd imagine it's pretty good.
Finally, you should know that most medical physics program are very small (some let in only a handful of students every year) and funding is much harder to get. There are fewer opportunities for TAing in the field and the cash flow is different.
If you're serious about medical physics, consider taking electives in biology, anatomy, computer programming, signal processing, physiology and applied mathematics (especially linear algebra and Fourier transforms). It is a multi-disciplinary field so the more background you have going the better off you'll be. And definitely get involved in physics research before you graduate and do one or more REUs.