I've got my 8800GT doing PhysX for my 260. I had to connect it to my monitor and extend my desktop.
It didn't make a large difference in "Nurien" @ 1280x. I moved from 66FPS to 72 with 4xAA|16xAF.
My old FPS in "Warmonger" at the tutorial start was 63FPS, that went to 79 @ 1920x. That's with all sliders to the right, or details maxed.
177.79 + PhysX 8.07.18 GTX 260 = 63FPS
As above but GT PhysX = 79FPS
So ya this will even work in an ancient Intel i975x board.