Yes you should be able to use a dedicated physx card for that game. How much of a gain you will see is debatable though and will greatly depend on what you are running as a primary and secondary gpu. Also, keep in mind that you will need to use 2 nvidia cards for this unless you want to try using custom drivers to make an AMD gpu work as the primary card.
FleX is a particle based simulation technique for real-time visual effects.Traditionally, visual effects are made using a combination of elements created using specialized solvers for rigid bodies, fluids, clothing, etc. Because FleX uses a unified particle representation for all object types, it enables new effects where different simulated substances can interact with each other seamlessly. Such unified physics solvers are a staple of the offline computer graphics world, where tools such as Autodesk Maya's nCloth, and Softimage's Lagoa are widely used. The goal for FleX is to use the power of GPUs to bring the capabilities of these offline applications to real-time computer graphics.