How interrelated are computational biology and biophysics?
Computational Biology is just the application of computers to solve problems in biology. It may mean using machine learning to identify regulatory regions in DNA or it may be solving a complex system of differential equations to model chemical reactions in some metabolic pathway. Whether a background in physics, chemistry, biology, math, or computer science is more central depends on the exact problem you want to work on,  and in fact which aspect of the problem you want to work on.


  Also, what field do my research interests actually fall under? I find
  it often depends on the university and specific department,


Bingo! Your interests are clearly cross-disciplinary, so for any given research question, different researchers in different fields will all be working on different aspects of the problem. Academic disciplines don't get to declare monopolies on research problems!

You Ph.D. is going to require you to spend 4-6 years working on a VERY specific problem. It would be wonderful if you could figure out exactly which research problem you wanted to dedicate the next 6 years to, and then choose the Ph.D. program based on their research program on that problem. This is rare though. More typically Ph.D. students have to spend a year or two doing rotations and talking to potential mentors before picking a research problem. In that case you are going to have to decide which academic discipline attracts you more, and then find a department in that discipline that seems to have a lot of collaborations with the other disciplines that attract you. See Buridan's Ass;