i had one publication where i was 7th (!) at the time of applications last year, 2 first author posters, and a couple of other posters where i was second author. i was accepted into 4 phd programs for psych/neuro. having the publications certainly helps you, but i think what is most important is how your recommenders can lavish your hard work resulting in the posters with praise, etc. talk it up in your personal statement too! i think what is most important is not that you have posters, but that you have a thorough understanding of the scientific process and that you're able to produce good work in your research area. this should be reflected in your recs and your sop, and should (i hope!) make you a stronger applicant. if you have these things, are a good fit in your lab of choice (have contacted potential advisor, etc), and you have luck on your side, hopefully something positive will happen