Ok.... I can think of it as a logical theory in case of both the replies here but there is a big gap since PhysX calculations are handles by the 9600 or 9800 card just on the GPU card itself (You'll need to visualize each card as an island with bridges connecting them) no CPU involvement here.....
The 560 is going to be lightning fast as compared to the 9800/9600....... BUT... they both handle different aspects of the game, one is Graphics and the other is PhysX.
They'll both do their job very well....
The PCIe lanes are sufficiently fast enough ad provide enough Bandwidth to both card for flawless work.
The slowdown is seen only during PhysX intensive scenes..... but that happens with all games, as the PhysX calculation cross a particular level the GPU's and Dedicated PhysX come under intensive load thus a slight slow down depending on the Cards used.
If you're 9600GT is the kind I use that does not need additional power, keep it in, it helps a lot, plus the fluidity of the 560 functioning exclusively just to render the graphics makes the whole setup a lot smoother.
If possible, we would like to see benches or fraps readings for you using the 560 as primary GPU and 9600 as Physx dedicated in the first scenario.
and in the second scenario have just the 560 in the rig with it handling the physx too.
The result should be clear enough. ( For all of us)
I agree that having higher end cards to handle both Graphics and dedicating the other to physx does help a lot.
But pairing a lower end card with a higher end card and dedicating the lower for Physx is not as bad as it may seem either.
It always helps to have a helping hand even is it is slower
