Quadro FX 6400? Do you mean FX 4600? I don't remember there being a FX 6400 out in the market. Anyway, it can work as a PhysX dedicated card, just make sure to use the Geforce Drivers with that card.
The only issue I can see is that the card may be a bit weak and may end up bottlenecking your GTX 470 when processing PhysX but I could be wrong since I'm not 100% sure how well the FX 4600 can perform.
This is a very good video to watch when deciding what is the optimal PhysX card for a powerful graphics card.
https://www.youtube.com/watch?v=cbww3dhzK0M
Anyway, you could also sell the card or use it as a spare/backup in case your main card breaks down unless you can sell it for quite a bit of money even though a bit old.