i dont know what you mean by "advances" in sciences but since you've already taken the premed courses (your usual intro to chem, bio, ochem and physics), i'd highly suggest for you to go ahead and apply to the SMP type of programs out there that cater to people like you. i graduated with a sucky gpa, did a DIY post back, and just finished up the SMP at BU and i couldn't have done it better (check out my mdapp if you want a better idea and pm me if you have q's). going back and re-taking classes isn't a great idea, since 1) it won't really help your gpa much 2) schools want to be able to see that you can take on difficult courses in the sciences to ensure your success as a medstudent at their school. even though your overall gpa won't be that great even after SMP, doing extremely well (think 3.8) in these programs will clue schools in into the type of student you've become and give them more faith to take "the risk" to admit you as their student.
gluck!!