Since you're in the overclocking section, you might want to steer towards the AMD cards. They tend to overclock a little better. The GTX 670 is still an excellent card though, overclock or not.
The GTX 660 Ti is a bit of a mixed bag because of its memory limitation. In some cases it serves up awesome performance and in others it's disappointing.
PS: PhysX is a proprietary Nvidia thing. AMD cards aren't supposed to work with it.