Especially if you have the batman series . It's the only way to run AK wide open with the dreaded Gameworks feature . I'm using a 760 for PhysX myself . Borderlands 2 however , is a different story altogether . That game just doesn't like modern gpus . Turn PhysX up to high &  have guaranteed system crashes , even with a dedicated PhysX card . Personal experience & other steam members can attest to it . If Gameworks would just go away &use normal PhysX a lot of games that have issues with it would evaporate away .

I tried a 970 for PhysX with my 980ti hybrid.  Only got 3-8 fps more on the games I tried.  Not worth losing the resale value of the 970. However, since the 670 is not worth nearly as much it may work for you.
 
Just try it out and see how it works.  Personally I would just run the 980ti. 

I think you can just use the CPU for PhysX?

EVGATech_AndrewA
Hello,
 
If you have an 980Ti then it would be unnecessary to have the 680 for physx unless you are using a game that requires physx such as Killing Floor.

 I would say this for Killing Floor 2 or the Arkham Knight game. Why not run it :P who knows there could more titles that support it down the future and it could get more demanding in the future with those newer titles, also it looks good in both of those games if you run it, if you want just let it idle in your system until more games support PhysX.
 
 
Also edit: It does not only affect 3 to 8 fps... I have ran a game in my own server for Killing floor 2 and lost 20 fps on my 980 ti, when I tested the game by adding a Phsyx card it helped a lot. Try having a huge hoard of gore all over you all alone and see if your fps is barely affected. It is a lot of computing with all the gore..

EVGATech_JamesL

deFiniLoGy
I think you can just use the CPU for PhysX?

PhysX is Nvidia's physics engine and would require a Nvidia GPU. Where as other physics engines such as Havok or Frostbites will use the CPU

 
Well not exactly. PhysX is Nvidia's physics engine. It runs on consoles, intel and AMD CPU's and can be accelerated by a Nvidia GPU meeting certain basic requirements. It all depends upon how it is implemented in the game by the developer. A couple of games allow you to choose a PhysX level where basic (low) PhysX effects run on the CPU and more advanced effects ie hardware accelerated (high), run on the GPU. Several titles see a significant increase in FPS using a dedicated PhysX GPU properly matched with your main GPU. Whether or not it is worth having the extra GPU for PhysX is subjective to the user.