The top 20 schools might vary based on your research interests but I would bet the top 10 will still be in the top 20 list for every subfield! Here is a US News ranking list and it's really for "Physics" and it's actually 25 schools (on the first page). However, I would say that this is pretty similar to Astro (although the rankings might not be in the same order).
The only school that I would consider in the top 20 for astro that is not in the top 25 on this list is University of California, Santa Cruz.
Note that some of these schools rank lower on this list because it is a "physics" list. For example, I would rank U Washington (Seattle) and Ohio State higher than they appear on this list. Similarly, some of the schools rank higher on this list because it is a physics list. Here, I would say that MIT, Stanford, and Chicago rank higher than I would normally place them in an Astronomy ranked list. These are still good astronomy schools and still would be in the top 20, in my opinion; just not necessarily in the high ranks they appear on this list.
As for GRE cutoffs, most schools don't really have very strict GRE cutoffs. US schools look at your application on the whole, so they will consider your GRE, GPA, any English test scores as needed, your research experience, what courses you have taken, why you want to attend grad school, and whether you are a good fit for the program.
For more information on how to apply to US astronomy grad school, you can consider reading this article: http://astrobites.org/2014/08/28/applyi ... -timeline/