i would like to know that too....
From my experience at a school where grading style and professor quality vary to a huge degree, the difference between a 4.0 and a 3.8 is ALL in the right choosing of professors (where possible) and one's being able to quickly adapt to different testing styles (that is, "testig well").
At my school, for example, students who take genetics either get a professor who is ungodly hard and whose tests pull lots of tricks (UT kids, you know who I'm talking about!), or one who is far less difficult and whose tests are more straightforward (there are a few of these profs teaching genetics here, as well). Going on sites like PickaProf will give you a good idea of whom to take a class with, and this is always reflected by the fact that the easier professor's classes fill up the soonest. Choosing the right instructor even applies to TA's in some classes (not labs, where it's standard to curve all the grades from the TA's of all sections at the end of the semester), if they are doing the primary teaching.
Also, the difference depends on your ability to be flexible in test taking skills (easier said than done!). If you're used to a certain testing "style," you'll find it more difficult to get A's in classes with a diff testing strategy.
But med schools, I'm sure, realize that there are factors involved in GPA that are beyond the student's control (like picking profs), so there really isn't much of a difference in two students' chances attending the same school, both with the same major, one who has a 3.8 and the other a 4.0.
If you're asking for advice on maximizing your GPA (assuming you already have good study habits and good test-taking skills), use online resources like PickaProf (most students here use this every semester when registering for classes) so you can avoid taking classes taught by "hard" professors in which you're less likely to get an A in. Also, if you can get your hands on old exams, this will help you understand how the prof asks questions (and, generally, how they "think" when writing tests). Trust me, these two stragies, I'm convinced, have helped me tremendously in the GPA.