I sat on the admissions committee for a US Med School for a number of years in the early 80’s. At the time when I applied, and for some time thereafter, about a third of all applicants were admitted. Of course there is a natural tendency to not apply in the first place if the chances are not good, so it’s hard to really give you a direct answer. The Medical profession in the US is increasingly less attractive because of various factors, so it’s also hard to absolutely quantify what the real competitive field is like.
As a rule of thumb, we looked at undergraduate MCATs as an initial gross screen, with grades being considered but placed into perspective against the rigor of the academic schedule and the college. We placed a great deal of emphasis on well-roundedness and non-academic accomplishments, trying to sort out the resume-builders from those truly interested in volunteer work or significant research.
Underrepresented minorities were placed in a separate category (and still are) because their performance on standardized exams and high grades in quantifiable disciplines such as sciences were so far below everyone else we would have ended up with no minorities unless we handled them separately. http://www.aamc.org/data/facts/2008/mcatgparaceeth08.htm
You can look at current MCATs and grades online for a given medical school or for the whole country if you do research, and you can also inquire into a given school to find out their policy.