gtx 680 vs 960

Solution
680 is still faster than 960 actually. The only advantage of 960 is it use much less power than 680 that you can get away with small PSU.

Now this is where it can get complicated. 960 is new so most likely the card get better attention by nvidia. Also there were lots of complain how nvidia gimping Kepler so they can sell Maxwell. That aside the way I see it Maxwell is simply more of a gaming card than that of Kepler. We see GCN based card catch up a lot and even surpased Kepler to certain extend (like 7970 which supposed to compete with 680 now able to catch up with 780). So having all console hardware using AMD gpu architecture might have it's advantage after all. Now more and more game engine get tweaked with AMD architecture as a...
680 is still faster than 960 actually. The only advantage of 960 is it use much less power than 680 that you can get away with small PSU.

Now this is where it can get complicated. 960 is new so most likely the card get better attention by nvidia. Also there were lots of complain how nvidia gimping Kepler so they can sell Maxwell. That aside the way I see it Maxwell is simply more of a gaming card than that of Kepler. We see GCN based card catch up a lot and even surpased Kepler to certain extend (like 7970 which supposed to compete with 680 now able to catch up with 780). So having all console hardware using AMD gpu architecture might have it's advantage after all. Now more and more game engine get tweaked with AMD architecture as a based. Right now AMD talk a lot about their async engine inside their gpu architecture. Nvidia probably aware about this hence they try to address the problem bit by bit with their own architecture. Right now Maxwell v2 (900 series) also have async engine like AMD GCN although AMD GCN probably still have the advantage in it. But by the time Pascal arrive nvidia probably can match AMD evenly in regards to async engine. Just like they have done with their GDDR5 memory controller.

Well sorry for the rant. But at 180 I think that still is a good deal. that 2GB might be a problem to some titles like Watch Dogs (though it might have been fixed by Ubi) and AC Unity.
 
Solution
Performance wise it is considered as mid range now. Still quite good for 1080p res gaming. As for how long it going to last it depends on how you like to play your games. If you're not picky about certain things probably it will last quite long. And finally DX12. DX12 might speed thing is up it need the program to use the API to take advantage of DX12 improvement. Also not all games going to adopt DX12 for certain reason.