Well , I would only do the switching back and forth if there was only one card , if there were two or more cards then I wouuld leave it at the Nvidia recomended setting. The reason I said that about BF3 is because it is a very graphic card intensive game and really makes the card stress out , so to relieve some of the stress of the card I would switch the physix to the cpu. It may very well be that the card can handle both but the only way to determine that is to run some benchmarks with the physix on the card and then on the cpu and see if the fps changes much if at all , I'm thinking it will but you never know. One sure way for you to tell is run the game with the Physix on the card and see what happens , is the gameplay smooth are there any artifacts , pixellation or any other problems and then run it on the cpu and see if there are any changes for the better.