I think the 660 (or AMD equivalent) is the minimum if you want to invest in PC gaming.
now more like a 660 ti is the minimum for somebody that wants to be into PC gaming. the thing is a 660 ti cost almost as much as a 670 and like i said a 670 with a slight overclock can match a 680 so still if you wanna game on a PC a 670 or 7950 (AMD equal) is the way to go.
I think the cards that end with 70, 80 and 90 as in GTX 480, 580 and 680 offer the better performance even though it may be in a lower series. For example GTX 480 is better than my 550 ti.
690 best card Nvidia offers its like 680s together
Titan best single gpu graphics card Nvidia offers weaker than a 690 though
680 best single gpu card outside Titan
670 second best single GPU card very close in proformence to a 680
660 ti third best single GPU card