In deference to MD/PhD programs, it depends on what area of research you are planning to enter. Engineering, biophysics, mathematics, or biostatistics programs will probably require some advanced math for your PhD years (CalcIII or Differential Equations are about the highest required, except for math). If you enjoy math and are decent at it, I would suggest taking the calculus series. Most schools to which I applied were impressed that I had taken such "hard" courses and done well in them. However, if you don't enjoy math and are planning on something less quantitative, concentrate your time on other courses...