The PhysX Card

Generally, a dedicated PhysX card is not required, but it is a great option. A dedicated PhysX card is a great option if:

1. You have an extra Nvidia card lying around
2. You are playing games with hardware-accelerated PhysX
3. Of course, if you have an extra budget to purchase a dedicated PhysX card
4. Your PSU can support the main card and the PhysX card

 

Here is a good reference of using GTX 650 and GTX 460 as dedicated PhysX cards:

http://www.evga.com/forums/tm.aspx?m=1743982