1) No. Regardless of the number of cards installed in your system, you can still choose to process Physx with your CPU. Or, you can turn off Physx from within the game.
Take a look at your Nvidia Control Panel, you should have the option to choose either your GTX570 or your CPU to process the Physx.
2) Yes. You can go into Nvidia Control Panel and select CPU to process the Physx. Most of the time, this is not a good idea because the Physx effects are usually too heavy for a CPU to process, thus resulting in much lower framerate. Take a look at the chart below. Using a Phenom II X6 to process the Physx effects of Mafia II, the fps is only 14.8.
A side note:-
For certain games (eg Mafia II), if you have a higher end SLI system (eg GTX570 SLI), the framerate will be higher with the SLI disabled (ie with master card doing graphics rendering and the slave card doing Physx) than with the SLI enabled (ie with both cards sharing graphics rendering and with one card taking up Physx as well).Edited by windfire - 10/24/11 at 6:53pm