do this, uninstall all your drivers, remove the gtx 260, and just install the GTX 460.
Then add the GTX 260 and select it as Physx card in the Nvidia control panel, sounds like it should work, right ?
I gave that a try and it didn't work. So, I just used a dremel and cut the bottom right corner off the ginormous plastic housing that they put on the graphics card. Now it fits fine, except one of the sata cables touches the edge of the fan occasionally.