The reputation requirements only changed once: when the site design was updated. Since then it's always been 3k points to vote to close.
As to what you should be doing, the best course of action is probably voting down questions with a comment as to why (or voting up another comment that explains the same thing you want to say). That follows the "community policing itself" idea that is what the SE network is striving for. I realize with the rep requirements as they are there aren't enough people with rep to actually do community closes anymore and it falls on the moderators to do more cleaning up then they should, ideally, but it's a start.
A flag isn't a bad idea, but they're just things that show up in the moderator panel. At worse we'll delete the flag without doing anything. At best it'll call attention to things that probably should be dealt with.