First off, congratulations on being so pro-active. Now is a great time to start looking ahead to graduate programs.
The most important thing for you to do right now is to make sure your GPA goes from a 3.34 to at least a 3.5 by application season. A 3.5 GPA and plenty of experience in the lab will get you into any program. Continue to work as hard as you can in the lab, but do not let your GPA stay below a 3.5. If you have a 3.5 GPA and a lot of research experience, your GRE only needs to be average.
Are you more interested in computational neuroscience or proper neuroscience? If you like computational, then of course take more programming. If you want to work on cells and animal models, take more biology and chemistry courses. Physics is also important for neuroscience since you will be studying what is essentially circuitry in the human body. I would not waste time with psychology courses unless you want to do systems neuroscience or behavioral neuroscience.
Finally, never retake a course in which you earned a C. Retaking the course may make your GPA look better, but graduate schools will still ask to see the original grade. I would just do better in a more advanced course to prove that you understand the material. And only take a subject test if it is required.
Good luck!