By default GTX cards are set to handle PhysX calculations, where available, however it is also possible to add a second Nvidia card to the system and nominate that to handle the non graphics workload. This leaves the primary card free to handle the 3D workload and should improve framerates. So, for example, in a situation where a consumer is upgrading from a GTX 280 to 295 the old card can be used as a PhysX processor. Shown above we have the framerates achieved when combining the 280 with the newer 285 and 295 and it is clear there are significant benefits to be gained from adding a second card. It does not have to be an expensive model which is added to the system though; a GPU such as the £40/$60 GeForce 9400GT performs very well in a game such as Mirror's Edge.
Giving statistics does not really help explain the benefits of PhysX and so below we have included a set of comparison videos which show some of the effects in Mirror's Edge with PhysX on and off. It should be reasonably easy to see the differences but some areas to watch for are the glass, blinds and debris which interact with bullets in the first corridor. The glass which rolls down the slope after exiting the building and sliding down part way and the smoke in the centre of the environment approximately 0.44 into the video. Given the performance of the GTX cards in this game there is no reason to turn off these effects in real world use, this video purely gives you a visual representation of what can be gained from running PhysX.
Download the file, extract the zip and run "RUNTHIS.swf" in a flash player, or drag to a browser window.