750 watts is nearly enough for GTX 780 Ti's in SLI, so you will have zero problem with a 780 Ti + 750 Ti + overclocked whatever else you want.
And apparently even a GTX 780 Ti on an i7-4770K can benefit from a GTX 750 Ti as a dedicated PhysX card.
http://alienbabeltech.com/main/using-maxwells-gtx-750-ti-dedicated-physx-card/
Quote:
"in every single case, the new GTX 750 Ti is stronger as a dedicated PhysX card than the GTX 650 Ti and we continue to see it bring performance improvements to the GTX 780 and to the GTX 780 Ti."