Shamus,
In a lot of ways, I'm in the same boat you are. I've got about sixty credit hours under my belt, but am consdidering changing my major from history (what I wanted to teach) to biology. That will mean I will need to get some chemistry courses pronto.
The world of teaching can be great -- I still plan to do it someday. But, the medical field seems to be the best bang for your buck. Take nursing, for example. You can get an associates degree in nursing, pass the test for becoming an RN and find a job with absolutely no problem. There is an incredible need for nurses, and more and more men are entering the profession. Get this: if you are willing to travel, some nurses can relocate and make up to fifty dollars an hour with rent paid for them!
Nurses with masters degrees can easily make over $70,000 a year. Another thought I've had is to become a physician assistant (PA). PA's need a bachelors or masters degree and can perform many of the same duties of a doctor. In some states they even can prescribe medication. And, of course, the schooling is far shorter and so is the residency.
The problem with becoming an MD is the extremely long schooling and INSURANCE. In Chicago, malpractice insurance costs some doctors as much as $200,000 a year! Now you know why going to the doctor is so expensive.
So, for me personally, I think nursing or physician assistant is the best bet. But, follow yyour dreams, by all means. The first year or so of school you'll be taking pre-req's anyway, so you don't have to finalize anything right away.
Go for it!
Bradley