Well, the 7850 is not "much worse" than the 670. It's hardly different at all. You may have had different settings on then, or some new, intensive setting may have become available with the installation of an Nvidia card. Nvidia and AMD use different types of specialized AA, for instance.
yea theres a big difference the 670 bf3 ultra=60+frames were as the 7850ultra= 40ish.
i know whats wrong its because AMD dont support Physx and nvidia does