Actually, most of the schools I dealt with calculated a pre-req GPA, not a science GPA.
Re-taking classes really depends on the schools you are applying to. Some average grades, which can make is less effective and more expensive. I did NOT retake any of my C's, but I did take higher level classes with better grades related to those subjects and I became a certified tutor in each subject I had a C in, which I think said a lot about whether or not I really learned the material. I understood what I was doing, but could not process the information fast enough to perform well in the exams (I do have a bit of dyslexia which also interfered with some of the chem stuff.) In my explanation statement, I included the direct contact number for the prof I had in chem/ochem and pointed out that no one obtained higher than a B+ in the class, and that she later recommended me for tutoring.
So, really, it depends on what schools your interested in, if you can afford it, if it will really change the results for the class, and possibly if you will have another way of showing htat you can grasp the material.
