According to http://www.nvidia.com/object/physx_motherboards.html, with "PhysX ready" motherboards, it is possible to dedicate another GPU to physX tasks. But is it possible to select intel's integrated graphics for this purpose?
And if yes, how much will it boost the graphics system if I would select core i3 550 integrated graphics for physX calculations, while using GTX460 SE for frame rendering?
It's a feature you'll never use. I know some laptops will run off integrated graphics to save power and only use their mobile GPU for 3D stuff.
The only integrated GPU that has a use even with dedicated graphics is the new Sandy Bridge EUs, and it's only going to be useful for video encoding as far as I know (hybrid CF aside, which is a very outdated tech anyway)