You sound almost exactly like me. I've always been enamoured by the stars, blackholes, quasars, and all of the other intricacies in space. I used to read a lot of Carl Sagan and Stephen Hawking back in high school/ early college. I'm even majoring in applied physics (double with biology). I had every intention of going into astrophysics, until I took an intro to astronomy and astrophysics in my second semester of freshman year.
It's not that I fell out of love with space and the concepts of relativity and the expansion of the universe, I just wasn't turned on by the practical applications of astrophysics. It's a lot more computer-based computational analysis, writing programs, and staring into space (the latter I could enjoy!). I just didn't have an affinity for writing computer programs or an interest in it at all. I sort of stopped with my whole astrophysics direction after my freshman year. The glamour of blackholes and supernovae and all of the goregeous pictures that you see in popular science books was completly stipped from the curriculum and replaced with spectra and blackbody radiation diagrams and whatnot. It wasn't even learning about the difficult theories that comprise popular physics or the fact that I had to take calculus through differential equations (not as scary as most people say, provided you have an english speaking professor)... it was the type of life I would have if I had decided to go into physics/ astrophysics.
I did, however, pick up a minor in astrobiology, which has been a hit at almost every interview I've been to. I decided that astronomy, astrophysics, and astrobiology are much better hobbies than professions (for me at least, I know a few people who will definately be successful in their astronomy related fields). When I'm a successful doctor, you can bet that some of my money will go to help out missions like the Mars rovers or cometary explorers!
Also, medicine has fields that you can use a physics background in (nuclear medicine, radiation oncology, radiology, etc) so it wouldn't be a complete waste of time. And it gives you that edge over the 50%+ of pre-meds that are biology majors!
But anyway... if you are into astrophysics or astronomy and seriously plan on going into medicine, I would suggest minoring in it. My double with applied physics and biology (and the minor) took me through multiple semesters of maxed out credit hours, tons of work and studying, and a summer taking organic chemistry. The people who major in astronomy and physics are generally the people who are going to be looking at grad programs later and to do summers at Arecibo and out in the desert. If you just want to fill your thirst for astrophysical concepts, a minor in astronomy will more than expose you to the current concepts and ideas.
And physics is a good major to choose
And a word of advice, if you do change your mind and want to go into astronomy, major in physics, not astronomy. I heard from my advisor that a lot of graduate programs in astronomy will take a physics undergraduate major over an astronomy major because they feel that the astronomy major doesn't focus enough on the physics concepts behind the astronomy.
Oh, and as far as prepping for the MCATs, not really. The MCAT really only covers material through physics II. There may be astronomy related questions, but all of the concepts will be, at their hardest, physics II concepts.