To my knowledge, Harvard doesn't seem to have many comp. neuroscientists. MIT CSBi(comp and sys bio) is a good comp bio program but I'm not sure about the neuroscience part.
The math score is good unless you want a more quantitative program like math or physics, but in that case you also need the subject GRE. GRE really doesn't matter much. I got into Harvard SysBio with a writing score of 3...
Being a gopher myself and highly involved in the micro community, I can't recommend UMN microbiology more. And keep in mind there are several very cool microbiologists who are not actually in the microbiology department but in EEB and Biotechnology Institute.
It's quite subject-dependent. Whereas liberal arts like history may require continuing reading and writing, for most science subjects slacking for few months is totally OK if you have had your courses solid.