Should I stay in my uni for four years even though I already have 50 credits? Since University of Alabama offers me full-ride scholarship, I will probably go to this school rather than more competitive schools, not because I'm poor but because it seems silly to me to pay $200,000 for merely an undergrad education, even though I can buy a house by that money. I'm going to get PhD, and the name of undergrad school doesn't matter in my career. I'm worried about research opportunity in the school since I want to excel in admission of PhD program, but I think I can make up for it with my enthusiasm and knowledge. I'm going to major in Biology, and I want to study about regenerative medicine and stem cell in grad school. 

Until I will graduate from my high school, I will certainly have about 12 AP scores (mostly 5's) and be able to get about 50 credits, even though the school's graduation requirement is 120. It seems easy to graduate within 3 years (or even possible to do within 2 years), but graduating early seems to put me in a disadvantageous position in grad school admission. I can probably get 70 credits in two years, and then what should I do for the next two years? Can I concentrate on research for this period, or should I take classes to get about 30 credits per year? Should I apply for grad program in the third year and try again in the next year if the admission won't be successful? 

If you have some opinion not only related to the topics about college credit but also my choice of school, please tell me that, since I still can change my choice of university. Other schools of my choice are such as Reed, Carleton, U of Michigan, U of Wisconsin, and U of Manchester. I'm an international student.
First of all, I know nothing about Biology at all. I can't comment on specifics. Instead, I rather say something about the comment:


  Do you think double-majoring using the available time gives me a significant advantage in my admission? Or should I just concentrate on the area which I will study in grad school?


You are far away from grad school yet. Undergrad education is for you to build the foundation of your academic career. You need to use it to broaden your knowledge base. Many courses can be benefit for you. You should consider taking the courses about humanities, fine arts, other sciences (math, chemstry, physics, computer science, etc.) and writing, etc.etc. Just don't limit yourself to a specific field. You won't know you're interested in something until you learn it.

The above is from the bottom of my heart. I wish someone would have told me this when I was an undergrad student.