On average, it will be harder to go from a little-known school to a "prestigious" school, but it certainly is not impossible. I think the key (assuming you have good grades, GREs, etc.) is to have good recommendations and at least one recommender who is really willing to go the extra mile for you (which of course requires you putting in time with that faculty member during your undergrad years). I went from a relatively unheard of satellite state school for undergrad, but had a great relationship with several fairly well-known faculty members there. I was able to go to a very good school for my masters (not necessarily prestigious, but very good in my field). And now I'll be going to one of the top-ranked schools in my field for the PhD. And I can say with very little doubt that it simply would not have happened had I not had the support of my undergrad advisor and other recommenders. Furthermore, if I hadn't gone to the small school I went to, I may not have had the opportunity to strike up such relationships.