You might want to spend some more time on the forum to find the discussion related to your question. There had been a lot of discussion on this topic in the past.
Also, I doubt anybody on this forum really knows the answer to your question( how much does the publication help). One thing we all know for sure is that having a publication even in some crappy journal out there NEVER HURTS.
Having a first author publication on PRL is not a joke. If you think you can do this, I can guarantee for you that you will get admission to much coveted top 10 Physics graduate school( check NRC or US news). (I am assuming that your GPA >3.0 and you have no criminal record
I do not know if I answered to you question, but my advice to you is you should at least "TRY" doing the following:
GPA > 3.6, well known schools (unknown schools, 3.9/4.0)
PGRE > 850
REU- 1 or 2
publication(of course 6th author in PRL, PR -A,B,C,D,E , Science, Nature, Nanotech is lot better than first author in some unknown undergrad journals. This does not, however, mean that you should ignore those undergrad journals)
All these are possible and NOT terribly difficult. Good luck.
Disclaimer: This is just a biased baised opinion of mine. That is what "I think" an applicant(domestic or domesticated international, like me) "need" inorder to be strong enough to compete with other students for grad admission in top-10 schools. Once a student fulfills all the above criteria, the rest of the admission process is just a CRAP SHOOT. Otherwise, nvanmeter would not be rejected from Stanford, quizivex would not be rejected from MIT and grae would not be rejected from Harvard (check the profile thread).