I didn't even know you could do this but I guess until recently, you could run a Radeon card for graphics and a NVIDIA card for PhysX in a Windows 7 system. Well, it looks like NVIDIA disabled this feature in a recent update
to the drivers though.
Personally, I think NVIDIA should've left it alone and let the consumers choose. If they would've left it in, there might've been more widespread adoption of PhysX and you'd still get people buying your card for those who are interested in PhysX acceleration. Now, they are forcing you to buy all NVIDIA hardware (minus the motherboard) if you want PhysX.
For me, PhysX is a hit or miss with products. Batman: Arkham Asylum
uses it well while I didn't find it that much more interesting to play with it on in Mirror's Edge