Adding a dedicated PhysX card can increase performance in GPU accelerated PhysX games by about 25% when a decent PhysX card is used. You would not use one of the GTX 580's as only a PhysX card. You would run both in SLI and have both do the PhysX calculations in addition to the graphics processing.
GPU accelerated PhysX games can use any GPU PhysX processor, whether that's the one single Nvidia card doing both graphics and PhysX; or one doing graphics alongside one dedicated to PhysX. So games have to be made to use GPU accelerated PhysX, but they do not need to "be made to use a physx card".
Batman: Arkham City s a good example of a GPU accelerted PhysX game that could benefit from a dedicated PhysX card, maybe even in an overkill situation like GTX 580's in SLI, but like I said, it's highly not recommended. The reason is that the dedicated card will use up power, even when not playing Batman, and add additional heat. You would also need a powerful, expensive mid-level or above 400 or 500 series card to get the performance benefit. All the same, the 580's in SLI should be more than enough to handle the performance impact of enabling maximum PhysX anyway.