Hey Taty,
I have not personally applied to the MD/HST program, but I know some people who are currently in and have graduated from the program. I would definitely recommend looking at the HST website (http://hst.mit.edu) and I'm sure you can get a lot of information about the requirements, the different programs, etc.
However, from the people I know in the program having a lot of research during your undergrad years is looked upon quite highly... The HST program in general is geared towards people who want to continue to do a lot of research (and there are different programs within HST, depending on your interests) and pursue medicine. The people I know are actually doing the MD/PhD program within HST. In general, it's a great program with great faculty (i have had the opportunity to take a couple of HST classes in my undergrad years) and if that's what you really want to do, i would say go for it! =-)
Hope this helps!