Nvidia bought Ageia... and this article speculates that the seperate physx card option is dead:
http://anandtech.com/video/showdoc.aspx?i=3224

GeForce 8 to get [b:8bd2ca48ac]software[/b:8bd2ca48ac] PhysX engine:
http://www.dailytech.com/GeForce+8+To+Get+Software+PhysX+Engine/article10706.htm