Should my references be famous if I want to get into top schools for PhD?
It's not necessary for your recommenders to be famous.  It certainly helps a little, since it's only natural for the admissions committee to be influenced by fame, but it's not really relevant to the quality of the recommendation so most people try not to be too influenced by it.  Instead, these criteria are more important:


Do your recommenders know you?  I.e., can they make detailed, substantive comments, beyond just listing grades?
Do they know what's required for success at the schools you are applying to, and what the competition is like?  Do they have a good basis for comparison in your case?  The best case scenario is if their students regularly apply and are at least sometimes accepted; then a comparison with past students is very informative.
Does the committee understand their personalities and writing styles?  Some recommenders are very optimistic and say everyone's great, while others lean towards understatement.  If members of the committee have read previous letters by your recommenders, or know them personally, then it's much easier to make a reliable judgment.  Otherwise, everyone will be at least a little more skeptical of the letters.


There are plenty of people who satisfy these criteria but wouldn't register as "famous" in the community at large.  On the other hand, if your recommenders don't satisfy them, then it will be harder to be admitted to a top department.  I wouldn't go so far as to say it's impossible, but this is worth keeping in mind when selecting letter writers.