No Po1nted is wrong. I had the 590 and added a 460 for dedicated Physx card. It did make a big difference, but only in 2 games that used Physx heavily: Batman AC & Mafia 2. So you have to decide if those 2 games are worth the money of an extra card. Yes there will be more Physx games in the future also.
Now Batman AC is by far the best use of Physx i have ever seen. When Physx set to high it is visually stunning, and once you see how badass it is you will not want to play without it. So when only the 590 and physx set to high it dropped framerate dramatically, almost in half. It looked like the physx took over a whole gpu of the 2gpu 590, this is even when physx settings was set to an unidentified core. (not sure if this was driver related and if it has been fixed by now, or just how the game was developed) So the suggested physx card for Batman is a gtx 460 as stated by the games delopers. After i added the 460 my framerate almost doubled, it completely took off the extensive physx load off the 590, which was bogging it down before.
So end of story is u have to figure out if it worth it to for only those 2 games right now?
I got my 460 for free so it was a no brainer for me.