Do you own an ATi card and have an Nvidia card installed in your secondary or tertiary PCI-Express slot so you can use it as a Physx card? did you also have problems actually getting Physx to run? the new 257.17 Beta drivers of Nvidia actually let you take advantage of a certain bug where you can utilize the Nvidia card as a side card for Physx while running an ATi card as your main card.
For Example you have a 5850 as your main card and a 9800GT in your secondary PCI-Express slot as a physx card, before 257.17 all you had was an additional card not giving you any better FPS but with the new bug in 257.17 drivers you can use that 9800GT as a physx card in games such as Cryostasis, Mirrors Edge and Metro 2033.
Nvidia is fixing the bug right now so i Highly recommend all you ATi users with Nvidia cards grab these drivers ASAP.