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.
.C.