Northwestern is a 7 win team over the long run. Similar to Duke, they typically only beat teams with losing records (with exceptions). I wouldn't read too much into Duke's win over them yet. Duke has Baylor next week (0-2) and Army later in the season. As is the usual case, they could win 8 games this year without ever beating a team with a winning record.
If Duke loses to UNC, that will tell you all you need to know. They follow that with Miami, @Hooville, Florida State, Pitt, and @Virginia Tech. I expect them to come into our game 6-4 at best. I think they'll lose to Miami, Florida State, Virginia Tech, and either UNC or Virginia (if not both). They finish the season against us and then @Wake, where they could lose again. They could win 8 games without every beating a team with a winning record. But I wouldn't be shocked if they completely crap the bed again and once again only win 4 or 5 games.