In general, I think 650+ is a good score on any SAT II subject test. However, if you're aiming to go Ivy League, I would say that 700+ should be your target. Keep in mind that it's perfectly normal to take the SAT II's again - I'd say, on average, higher-level students will take 2 SAT II's only once and at least 1 SAT II twice (I hope that makes sense.).

I don't think there's a cutoff. Plus, you must keep in mind that most schools are also looking for strong SAT II scores in a few other non-scientific fields, such as math and English. I think high SAT II scores in both of the other areas would certainly offset just a good SAT II score in science. Also, take into account the school you're applying to. If it's MIT, you can bet 650 in science is not going to cut it. But, if you're aiming for Cornell, then things might be different.