No such site actually exists, as far as I know. A few of the programs give guidelines on their websites, but most do not. Some people have had success calling the admissions offices of the schools that interest them most and asking them directly what a typical GPA/GRE is for admitted students.
And, as has been mentioned before but is worth repeating, don't automatically be deterred if your GPA/GRE are a bit below the average for a given school - good experience and clear goals may still get you in.
Good luck!