I recently purchased a GTX 670 in the hopes that it would speed up my games a bit (A lot, I hoped).
I have seen no such increase. In Sleeping Dogs, with everything maxed out except antialiasing, I get 30 FPS on average. I see benchmarks people post in forums and they get that WITH antialiasing maxed out. They usually get twice my FPS on average.
i5-2500k @ 4.8 (1.36 vcore)
8GB ram @1600
p8p67 motherboard (kind of old, kind of cheap)