Don't get a 460 for PhysX. Too few games use it and those that do can manage with a 560 Ti alone... Mafia 2 is maxed out with all PhysX on my 560 Ti and it's butter-smooth. If you just itch to have a separate PhysX card just for the hell of it - which I totally understand, its fun to fill all the slots on the machine and enable all kinds of stuff! - then find a used 8800GT or something similar to take for free from a friend or get a really wek card that will be fine for PhysX, but that's a waste of money, IMO.
Thanks, that was going to be my plan for my 9400 GT if I didn't need a beefier card for PhysX, I wont be doing anything else with it once I replace it.