If you can, find someone who knows the program better than you do. Ask questions not only about the quality of the program, but also about the politics involved. Be sure you know what you're getting into. Some programs are internally very competitive for assistantships, PhD programs, etc. Other programs are nurturing and collaborative, and students actually help each other out.
Funding is also important.
Find out your chances for an assistantship. Some programs offer lots of funding, others very little. Don't pick grad schools based solely on rankings. Any information available on a program is probably useful, but these rankings often reflect a school's reputation, rather than the actual quality of the program. In any case, your criteria will probably be different from the ranker's.