pbehzad,
Congratulations on doing some planning for your future. Do some research on dentistry before you blow money applying. If you really want to be an MD and you don't get accepted this year, then I would strongly encourage you to apply again. If you really want to do it, there is a way.
As far as dentistry goes, scores that will make you extremely competitive (read: interviews at nearly everywhere you apply & a good deal of acceptances also) are a 3.5-3.7 overall GPA and a 3.3-3.6 science GPA. Those fluctuate a bit, of course, depening on your extra-curricular activities, proof that you are interested in the field, etc. DAT scores would be 20's overall.
There are no different classes for premeds and predents, however the DAT does not test on physics, so this is often delayed until the applicant's senior year. If you've studied for the MCAT you can do fairly well on the DAT, however you will find that the testing styles are polar extremes. If you are really interested in pursuing this, I'd suggest you take a look at the Kaplan DAT prep book (local library, bookstore, etc.) and take a gander at some of the practice exams. Kaplan.com also offers a practice exam so you can become familiar with the question types and different sections. The DAT does include sections which aren't on the MCAT -- but you'll find out more about those as you look at mock DAT tests.
I hope this was somewhat helpful -- I didn't go into extreme detail because I didn't get a sense that you were very serious about pursuing dentistry. I may be completely incorrect on that topic (apologies if so), but it seems as though dentistry is a backup resort for you, which it shouldn't necessarily be. If you research the field a bit more you'll find out how it suits you (or doesn't suit you). If you are pursuing it simply because you can't get an MD you might have a very miserable career! Dentistry is a thriving aspect of the health-professions, but there are a key number of differences between the DDS and MD, and sometimes those differences poke and prod people throughout their entire careers. That could lead to a very unsuccesful and unhappy career.
On the other hand, perhaps dentistry is right up your alley! If so, congratulations! You'll be entering a field where the pay & hours are far better than 98% of MDs could ever hope for. You'll also be likely to experience a level of autonomy that relatively few MDs ever experience. Dentists (and predents?) are a close bunch and enjoy sticking up for each other. For example, 20% of MDs are members of the AMA, while 85% of DDS/DMD are members of the ADA. Dentists are in control of their profession and the sky is the limit.
Good luck with your decision!
-G
