I ran an 8800GT with my HD 6970 for PhysX. It's easy as pie to setup, Google it and download the modded Nvidia drivers and then it is a matter of changing the names of a few files in your game and it works. I would expect a GT 640 to do OK, but something like a GTS450 or GTX 460 would be much better.
As for using that 580 for Physx, waste of time. The 670 can already handle everything just fine, it may even slow down things with the 580 because of the crosstalk. I wouldn't bother.
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