With a GTX 780, a GT 640 will improve your framerates somewhat in GPU accelerated PhysX games. You would need to decide if the additional heat and noise is worth it. The GTX 690 in the chart below and the linked review is even more powerful than a GTX 780, so the 640 will be an even better match for the 780. Check out the difference between "No PPU" and "GT 640".
Using Maxwell’s GTX 750 Ti as a dedicated PhysX card
http://alienbabeltech.com/main/using-maxwells-gtx-750-ti-dedicated-physx-card/
Dedicated PhyX PPU / GPU Shoot Out: GT 640, GTX 650, GTX 650 Ti, and GTX 680 – paired with a GTX 690
http://1pcent.com/?p=169