the only help it would give, as you may already know, is in Nvidia supported games that include "PhysX". Other games will see no benifets including your wallet as there is no way to shut off the power to the second card.
Will it help in games with PhysX? Most definitely it will help in those games and increase performance.
Is it worth putting it in? This is something you will need to chose for your self. Just remember that your PC will draw more wattage so be sure you have a big enough PSU to support both the 780 and the 555 at the same time.