I took the October 15th subject GRE, and scored an embarrassingly low 620 (43%) on it. My application is otherwise quite good (honors student graduating summa cum laude from well-known state university, teaching experience, good general GRE scores, several graduate-level math courses, an REU with publication in progress, 3 great letters of recommendation), but I'm terribly worried that my score on the subject exam will bar me from the schools I'd like to get into. There isn't any possibility of retaking it, either.
What sort of school should I aim for? Do I have any chance at getting into a top 20 school now?