"current" games havne't been written much with PhysX since the mass availability of the accelerator hasnt happened yet. I have an 8800 320 and just ordered a 4850--i think i'll give it a go with Vantage to see if thre is a difference.
I've successfully run a dual monitor setup with an Nvidia AGP card and an ATI PCI card, with both sets of drivers installed--so to say "it won't work" might be a little short sided.
I agree, I don't think "it won't work" is the right answer. I don't really think there is or will be an easy out-of-the-box solution to this problem. I'm not really sure that Nvidia would create a driver to make one of their cards look solely like a physics card. At the same time, if you really understand how the drivers work, you may be able to gerry-rig something. Just make sure that the display drivers default to ATI and have the Nvidia physics acceleration drivers... or something
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.