darkbreeze is right.
What you're comparing here are cards from two different series, the 760 being from a new series. You can call them generations if that helps make any more sense out of it. Both AMD and Nvidia do this. In this case, the 760 would generally be inherently better than the 660 because it's newer. Both AMD and Nvidia introduce new technologies with each new series and some of these technologies cannot be taken advantage of by older series. The 960 is the newest of all three and, you guessed it, that makes it better than both the 660 and 760. The only way an older card could beat a newer card is by being faster, usually through factory overclocks, but that's another story. The one darkbreeze linked to also seems to be competitively priced, so it's definitely the your best choice. If you ever want to compare two specific cards, here's a tool for it:
http://gpuboss.com
Something else I should probably mention, the first digit in a graphics card's number is the series it belongs to, and the higher that number is, the newer it is. The following digit is it's level in the series, basically, and the higher that number is the more powerful, and consequently more expensive, that card is. If you go looking at AMD's cards and get stumped on how they classify theirs, just say so. They have a rather different way of doing it nowadays.