Who knows... has anyone ever tried this kind of GPU brand mixture? Ain't there an inherent driver conflict between Catalyst and Forceware?
If the drivers won't collide and kill each other then I guess the performance boost, in something like Batman Arkham Asylum, would be notorious given the fact that it is, as far as I know, almost unplayable in an Ati card with Physx activated.
In such scenario, the 9800GT would solve the Physx conflict, allowing the HD to deal only with graphics which I guess would give steady 60fps with full Physx.
Actually it depends. The 470 is better than a 5850. Without PhysX it wins. There's not many games that have PhysX acceleration, but I've played several with a dedicated GT240 (close to a 9800GT, maybe closer to a GTX) and the usage is usually quite low. Maybe 15%. The one exception is in Batman AA, where it's at times been as high as 50% due to all the cloth and smoke PhysX stuff but only at some scenes.
I think in general the GTX470 is going to be faster even in PhysX games. Batman, the 5850 + 9800GT should win.
Although, if you're asking because you have a 9800GT laying around, you can use it as a dedicated PhysX card with the 470 too.