i was told by applicants the year ahead of me that you should aim for at least a 1200 (ortho) - but there is not really any posted minimums for the schools I am looking at. from what I can tell, it seems like about 1/2 of ortho programs require it, and a handful of programs in other specialties... but the MS doesn't have anything to do with it; for example OU gives an MS but requires no GRE, while every school in texas does. go figure.
I would reccommend the book by Barron's - their vocab list was right on! I think a few weeks in May/June would be enough to spend on the GRE if you want to be able to send scores out about the same time as you are sending in applications.