GPU physx really shines here..just as many imagined it would vs CPU.
"In SLI mode, both GPUs take on the physics and graphics workload equally. In Multi-GPU mode, on the other hand, the primary GPU (like a GTX 280) is dedicated to 3D graphics while the secondary GPU (like a 9600 GT) performs only PhysX calculations. This mode will be supported on both SLI and non-SLI motherboards. "
http://hothardware.com/Articles/NVIDIA_GPU_PhysX_Pack_Preview/?page=3
"In SLI mode, both GPUs take on the physics and graphics workload equally. In Multi-GPU mode, on the other hand, the primary GPU (like a GTX 280) is dedicated to 3D graphics while the secondary GPU (like a 9600 GT) performs only PhysX calculations. This mode will be supported on both SLI and non-SLI motherboards. "
http://hothardware.com/Articles/NVIDIA_GPU_PhysX_Pack_Preview/?page=3