In my opinion you got yourself an upgrade. The 960T's are better assuming you can unlock it to 6 cores, however theres no guarentee of that. the 965 x4 is a very good CPU for gaming. Yes, indeed I would overclock it.
For starters even without an aftermarket heatsink, you can set the multiplier to 18.5x, this would give you the stock settings of a 980 at 3.7GHZ.
As far as the gray area between the different models. I can say that most of the Phenom II x4s should be able to hit 4.0GHZ, I wouldn't expect any higher than 4.2GHZ, but it does happen. Even CPUs of the same model are not all the same, even intel's this applies. For example, you could have two 965's and one might be able to hit 4.4GHZ, and the other might never see anything above 3.9GHZ. You just have to see what yours can do. Me, personally, mine was "Stable" at 4.2, but my cooler isn't good enough to keep it at acceptable temps (by my standards) any higher than 4.0.