As was noted higher up in the thread, non-traditional students often do have an advantage as far as not being required to hold the same GPA as "traditional" matriculants for two main reasons: first, life experience does count for making a more diverse and interesting student body, and admissions committees often will take that into consideration during their deliberations. Second, nontraditional students often do better in graduate school than their straight from undergrad counterparts, because they know what they are sacrificing to come back to school and know what it means to commit one's self fully to a career. Be sure you make it clear in your application, however, why you are changing directions and that you are wholly committed to your new path. You should not duck responsibility for your less-than-stellar performance as an undergrad (especially if asked), but also highlight how that was the "old" you which has little to do with the new, responsible, successful you.
Another point: when it comes to grades, many schools are most interested in your last two years of school, so if you "straightened out" after sowing some wild oats, that will be weighed into the mix.
Taking additional coursework won't erase the past, and it really depends on the rigor of your additional courses -- for example, when it comes to medical school, enrolling in a formal pre-med prematricuation program (where you're essentially taking the first year of medical school courses) would be seen more favorably than, say, taking a few extra science courses at the local community college. Also, supplementing academic work with experiences relevant to the new field (for med school: shadowing/volunteering in the community for health-related activities) would also be seen positively.
The admissions director of the schools at which you have some interest can often give further insights, and they are accessible, but be sure to have questions that run beyond what can be found on the school's admissions web site, and make sure you don't hit them during admissions crunch time (just after the break would be ok, but as you get into March, things get hairy).
Good luck.