When the first physics cards came out they worked with your video card. After nvidia took them over the drivers they came up with make the stand alone physics card useless. For my money I'd go with the 9600gt.
I just put the physics card back in right after I read this post to make sure. The physics option when using the nvidia card or cards was 3 times faster than trying to use the bfg physics card with one or two graphics card. What nvidia did I have to assume is put the consumer in the position of having to buy their product ( another video card ) if you want to have a separate physics card. IMO you don't need a stand alone card yet. It will run fine on a good graphics card ( single or sli). At least that's the performance I'm getting.