I'm doing it right now. I've got a 5870 as my primary and a GTS250 for Physx. The process has even gotten easier in recent months. You just install both drivers and run the PhysX patch and you're off and running. Previous versions required you to have spoofed multiple monitors and were a little wonky. Now it just works. You don't even need the Nvidia extra software anymore.
I bought a new graphic card, ATI Radeon HD 5450. My current video card is ATI Radeon HD 3850.
When I plugged my new video card into the 2nd PCI-E slot, my computer didn't recognize. What I mean is my computer doesn't even recognize that a new device has been installed.
I am currently using Windows 7 64 bits and my motherboard is MSI P6N SLI Platinum LGA. Yes I know, my motherboard is Nivida SLI capable but I just don't need it so I decided to have 2 ATIs card instead.