ha haha, it's hard to decide where to start. I'll apologize in advance for my cynical view of the process; my opinions are clearly being colored by my recent lack of success in the application process. If you are able to hold your own in science classes and if job security/decent wages are extremely important factors to you, then I would suggest that you do medicine. I'm in love with the research aspect of clinical psychology, but I know that most researchers have to move around the country several times, can have difficulty finding positions (especially if you haven't published a ton as a grad student), and maximum wages are typicaly less than the average salary for a physician. Usually your bachelor's in psych is enough (esp. if you are coming from a big school), but each program has certain requirements that they are looking for, so check ahead. You will need to take the general GRE, and for most programs, the psych GRE as well. Research is looked upon VERY highly, particularly at the big research institutions. Volunteer work is also important to show that you can handle dealing with clinical populations. If you only have time to do one activity, however, definitely do research. The average GPA for admissions is high (at LEAST a 3.6 at the top programs), and you should have a GRE of over 1300. On top of lots of research. I have heard that getting into psych grad school is statistically more difficult than medical school. As for the top programs, there is a thread on here that lists the US News and World report rankings. Don't worry too much about the specific rankings, but if you are interested in pursuing an academic career, make sure you go to a somewhat high ranked school. Look at the schools that your professors went to, as use that as a guide. Just my two cents' worth.
Feel free to send me a message if you have any more questions or want to know more about what I did during undergrad. Good luck!