Everything I have read says that PhysX cards are a waste right now. Could change in the future of course. I looked at buying one till I did the research.
I doubt the dualing video cards would work right though. It is nice in theory. Probably not so good for games. If you wanted to write some custom code to do computations of some sort it might work well using the 8800 GPU.
ATI is working on adding its own physx engine as well, don't forget.
I have dual 9800GTX+ SLI in home gaming machine and a 4850 in my traveling Lanbox, so I don't have a bias.