I don't know what you're doing to get that high of FPS out of a 8800.... either way, a 8800 is the the 'fulcrum' point of physX cards. At 112 CUDA cores, you will get acceptable PhysX results but it's going to be a bottleneck on many PhysX titles (for getting a smooth 60fps). For mafia 2, I really suggest you run on Medium PhysX. If you don't trust me, watch my youtube videos comparing High to Medium PhysX,
YouTube - klizilii's Channel . I've had a lot of driver related nightmares with my GT 240 PhysX card (changing my dx 11 to dx 10.1, no SLI scaling in Just cause 2, etc)! The best (I find) is to install the main GPU(s) and then install the 260.99 drivers. THEN (with your computer off of course) plug in the PhysX card and just boot your computer and set it as a
dedicated PhysX card in the NCP. I did not find any loss in performance by using this method and was able to keep the dx 11 status of my computer. Still, even as a bottle neck (the 8800 can't do PhysX as fast as your 460 can do graphics), your 8800 will get you better Mafia 2 PhysX results than just the 460 doing graphics + PhysX.
My 240 runs at 50-58% usage when running the insane computer breaking benchmark with PhysX enabled.