I published two papers and presented at a conference during my undergrad. In each case, though, it was a situation that was specific for undergraduate students (i.e. my university's undergraduate research journal, student conference, etc.). Although both of the papers that I published are somewhat related to what I want to study, they are light enough and distant enough from my future subject matter to keep me from worrying about being "weighted down" to undergrad-level ideas (one was about Disney's Beauty in the Beast, hehe).
Ultimately, I'm not sure that they made much of a difference in my applications--the schools that I was accepted to commented more on my writing sample. Do it because you enjoy it--I certainly had a good time!