Arma use PhysX only on CPU, thats what the devs said, thats the final version, that is the truth. I'm using Radeon R9 270C (2GB) and i5 4690 (4 x 3,5Ghz) and have same much FPS as friend with 2GB GeForce (dont know exac model) and same CPU. We bought PC's in same time, with every part the same except graphic card (diferent models, but same parameters) - he is mad till today he payed more (much more). I'm not a fan of AMD, i allways used Nvidia, and i was afraid to buy AMD, but many users was calling this card best in its price, and i can trully say, it was worth it.
As for physX itself - some game (maybe GTA V - i dont remember) installed physX drivers on my PC (even i got AMD :D ) and There was no diference in FPS at all.