You will only benefit if the game is coded for Physx. I know 200 series has it onboard. You might remember the Aegia PPU card, Nvidia bought them in 2008. Dedicated PPU's didn't really catch on, but this is the same technology incorporated directly into the GPU. Multi-GPU SLI takes it off the main GPU and puts it on a lower end card, typically gaining around 10 fps on average from the benchmarks I've seen. 8 series and up has this ability. Personally, It's not something I'd invest in another card for, but if you've got one laying around you might as well run it.
http://www.nzone.com/object/nzone_physxgames_home.html