Years ago, the recommendation I remember is that to be at all worthwhile, a PhysX card needed to have at least 100 CUDA cores to make a difference. As powerful as modern cards are now, I'd like to see that tested again; I'm inclined to agree with TV that they are likely pointless, although it would be interesting to see if throwing in an old GTX460 or GTX560 for PhysX is worthwhile. This is rather drifting off-topic though, except that you'll need to be sure your PSU can handle the additional card.