From my understanding (and when I applied) psychology is kind of a gray area, so for your application to any of the tri-councils (although I'm not sure about NSERC) you need to write a statement justifying why your research fits within their mandate, so its good to look up their policies on their websites and see where your research fits best. I'm in a clinical program and I do know people who hold NSERCs.
As far as grades, they look at your last two years of work. I'm still waiting to hear, so I'm not sure how hard they are to get
Correct me if I'm wrong, but I think CIHR tends to fund more gender-related research and health stuff. I applied to SSHRC and I know I tried really hard to make my research intentions sound as social-sciency as I could! If you are in a lab, or know professors who've applied for their own grants, they usually have a good idea of which council would be your best bet.