Which GTX670 is it, out of curiosity?
When the GTX670 and GTX680 were the mainstay of Nvidias high end, conventional wisdom was that you bought a quality GTX670 as it wasn't a million miles away from the performance of a GTX680 and dramatically cheaper. Now, neither is widely available so you are shopping from stock clearout, prices don't really compare in the same way.
In most games, the difference is likely to be a few frames per second, it's pretty small.
In reference to the current Nvidia models, a GTX770 is a slightly improved GTX680, and the GTX760 falls somewhere between the GTX660Ti and GTX670. All five are variants on the same GK104 chip, but in most games the top to bottom (GTX770 to GTX660ti) is probably only covered by 15fps or so - again this is hugely dependant on what you are playing and the settings you are running.