I would not bother with their official information about the cut-off line. It is simply the score at which you stand zero chance and are automatically refused admission without any consideration. From what I have read in other threads, a Q below 160 may conceivably damage your chances, and anything below 156 or so makes admission unlikely.
For a peek at realistic GRE results for admission, I suggest checking the results page. Northwestern has a bunch of recent admits whose Q is 158-159, but not below that. I'd also search for any mentions of Northwestern and other relevant schools in older application cycle threads that tend to outline the profiles of the applicants, including their GRE.
I get the impression that GRE scores are steadily creeping up, unfortunately. You can see it well with Duke - a higher-ranked program, but it looks bleak, and it seems reasonable to assume that the situation is mirrored elsewhere.