Honestly there isn't very much distinction between a 670 and a 680 unless you're buying a reference 670; most non-reference 670s use 680 PCBs, so the difference between the two usually comes down to out-of-the-box clock speeds and the fact that even a high end non-reference 670 will cost significantly less than a comparable 680. 760 is about at the point of a reference 670, but not a 670 that you should actually be spending money on! At the moment you can get non-reference 670s for around $250 on Newegg making them about the same price as a non-reference 760; I'd go for a 760 just because it supports Boost 2.0 and stuff whereas 670/680s do not. A lot of it comes down to that sort of stuff, which you can't discern just from the name. Unfortunately naming schemes for all of the current gen GPUs are quite arbitrary that way. AMD is totally worse, though.
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