I think Computer Science, Statistics, and Applied Math are your best bets. By far.
You aren't going to need abstract algebra, topology, or real analysis for computational neuroscience. Honestly, it's far more important to learn things like the conjugate gradient algorithm, data mining, and convex optimization
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And honestly, many computational biologists are people who majored in technical subjects who then self-studied their biology/neurobiology along the way. You don't need a huge amount of biology for computational neuroscience at all - in fact - I worked with a professor in computational neuroscience (he's actually an applied math professor) who doesn't even know organic chemistry. I was able to get a 3.7 in a graduate level neurobiology course (Biophysics of Nerve, Muscle, and Synapse) even though all I had was a self-studied 5 on AP Biology (unfortunately I couldn't take upper-division biology courses due to my school's strict pre-requisite system for biology courses). You'll see lots of people without biology backgrounds in those fields (in fact, most computational biology professors at my school come from physics/applied math/CS backgrounds - almost none come from a pure biology background). Sure biology is *very* interesting, but if it takes time away from developing better analytical skills (or diversifying them by doing a double/triple major in Physics+CS+Stat/Applied Math OR doing grad-lvl courses in those fields, which are often TREMENDOUSLY useful for computational biology), then it's better to develop the analytical skills and then to do research with a comp bio prof. In fact, biology students have notoriously poor analytical skills, so you'll have a huge advantage over them by simply pursuing computational biology.
IMO, a directed self-study using Christof Koch's "Biophysics of Computation" or Abbot's "Theoretical Neuroscience" book would do a lot more than doing a biology major.
If I went to Caltech or MIT, then I might consider a biology or CNS major (in Caltech's case) or a brain and cognitive sciences major (in MIT's case). At least I'd then know that the biology I'm learning is going to be analytically rigorous. But I don't go to those schools so I'd prefer to stick with applied math or physics.