Hmmm what a odd combo, usually people just run physx off the main card or on a weaker nvidia card than your gts260. Your gtx260 should handle physx on its own if not track down a 8600gt or something along those lines and dump it into the second pci-e slot if you got one. If not check the agia card to see if something isn't right like a missing cap or burn marks. Clean out any dust if any is there.
The phys-x drivers are part of the Nvidia Driver itself, so when you down load the newest Nvidia Driver for your GTX 260 the phys-x driver is already included.
Do a normal driver install of the Nvidia driver and you will notice a Phys-x setting in the Nvidia control panel menu (as long as your card supports phys-x of course)
I've never once had to download a separate driver for phys-x.
If you want to see something cool download the phys-x screensaver and watch it go. I've got a family picture as a flag and it's funny to see the phys-x ball tear it apart.
You can download the screensaver from the driver page at the bottom of the