Any drivers newer than 186 will not allow a dedicated PhysX GPU to operate in the presence of an ATI card. I recently went through the headache of getting my 4870 to play along with a 9600GSO, but took the gso out and put it in an HTPC parts pile. When I did get it to work with older drivers, while very pretty, it was so punishing to my framerate it wasn't worth it for me(went from 75+ fps in Batman AA to 25ish with PhysX being the only different setting) perhaps using a better nvidia card (like a 275/85) as a dedicated PhysX card will prevent this sort of crippling, but that was too rich for my blood. Add on top of that, if you choose to only use old drivers (as i had to, xp64 would not allow me to use the hacked ones) then you are basically frozen in time as far as PhysX drivers go.
I'm glad I only paid $30 for the GSO, heh, and I have another home to put it into.