The physX cards are a dead end. The company behind them, Ageia, was bought by Nvidia some time ago. Since then Nvidia implemented the cards features on their newer graphic chips.
You should google the games that support Ageia Physx. There aren't that many and the difference between having a card for it and not having a card for it are minimal. A better graphics card usually has a far bigger impact on gaming performance and offers better visuals too.
If you really want Physx get a 9800 GTX or wait until nvidia has done the drivers for the 8800 series and get one of those - whatever fits your budget. That way you get your physx card AND a better gpu.