sonicmascis

The accepted student's average is generally around 1400, 4.5-5.5 AW, and (I'd say) 3.7-3.8? If you're around that ballpark then I would say that everything else will just depend on your fit with the program in addition to having strong letters of reccs.

It's hard to tell exactly what schools are looking for, but I think you could be a strong applicant. Your grades are a little low, but your GRE scores will be fine. You should definitely focus on research over teaching, or do both if you can handle it! Your presentations and publication are going to be the strongest point in your application - it is very rare for undergraduates to publish articles by the time grad school applications are due.
The only thing to keep in mind is that Columbia and NYU are great schools in great locations, and as such they get TONS of applications every year. To really set yourself apart from other candidates you need to have research interests that match not only what professors are currently working on, but where they are hoping to take their work in the future. Good luck!

To your #2 question, I would say Research assistant. TA at undergrad level is kinda mehh, but definitely way better than not doing one (like I did). Please get your self an RA position if you can!!! I wish I did that.I really like NYU's Psych department. I don't know about their MA prog, but their BA curriculum is VERY progressive in my eyes (non-clinical). But Stony Brook might be nice also, I know a lot of their professors are clinicians. One there ,forgot her name, did some ground breaking stuff on pattern recognition in the cog dev areas.