Radeon + Geforce on a system..

It'll work in theory, but there is possiblity of horrible conflicts. ATI and nvidia drivers don't play well. Most people who jump on physx are nvidia fanboys who get two nvidia cards, so there's no problem there.

Do you already have the nvidia card that's to be used for physx? If not consider getting a dedicated ageia physx card instead. It's out of style, but would certainly prevent any driver problems. That is, if physx is that important to you.

PhysX is not simply for fan boys and dual card users. It works quite well on a powerful single gpu. Now that it is so wide spread it could quickly become relevant. I'm really hoping that the physx on ati thing works out because I intend to be moving to a 4870x2 soon and don't want to lose the benefits I've had in ut3.

Yeah.. i thought about the driver conflicts.. >.< Damn.. Hehe..
No, i don't have a Nvidia card.. And the Ageia physx card barely exist here in sweden, and if i found one it will probably cost 2-3 times more than a GF8500 (Stupid swedish prices >.<) Oh well.. I can live without physx.. It's not that important for me.. Just a cool thing to have.. haha  

http://www.nordichardware.com/news,8035.html

PhysX is not just GPU dependent though. It may be accelerated due to the parallel nature of the GPU it's running on, but it still suffers from heavy dependence on the CPU.