I have two GTX 460's. I also tested them for awhile with several different cards used just as PhysX cards.
I can tell you that the 9600GT (MSI 9600GT 512MB) worked the worst out of all of them that I tested.
Actually all of the cards that I tested really did a worse job then if I used the on board PhysX from my GTX 460's.
The only GPU that didn't do worse (Was more about on par) was using a GTS 450 lately. This was done by using my buddies GTS 450 and overclocking it.
I would suggest just using the GTX 460's it's self for PhysX. If you are playing games like Batman exc..... one GTX 460 is enough to play the game. Then use the other one as the PhysX card.
I did most of my testing with Benchmarks though.
But the only time I got really any benefit was if the game was too much for one GTX 460. For example Metro 2033. Because I needed more power from two GTX 460's to play that game that is when the 9600GT came in handy for PhysX.
But still it didn't do very well.