As you all should know by now Battlefield 3 does not support the option to enable Nvidia PhysX so this makes it so the game does not use the such technology.

If you use say 2-way or 3-way SLI and have a separate dedicated PhysX card such as 2-way SLI GTX 570's and a GTS 250 for PhysX, the GTS 250 will not run just the 570's.

So it comes down to its best to run the PhysX option when running SLI than to have an extra graphics card dedicated to PhysX.

If you have AMD graphics you don't have to worry about this problem as AMD includes their own PhysX technology called Havok and is built in and enabled in all of the Radeon HD series cards!