I think if you already have both cards go ahead and test it. I think it is useful to have a dedicated physx card if you plays games that truly take advantage of physx which I haven't seen to many now days. I was thinking about trying my old gtx 580 as a dedicated physx card for fun with my gtx two 780 Ti's I don't really see any issue with just putting one of them in there just to test it out. The 9600 gt was known to be better at physx than the 8800. I might test it on my system with the 580 like I said. I also have a 650 ti laying around that I might test it with also
The old G80 based 8800gts isn't a good choice for physx and it has issues running the latest versions, the G92 has good support and is good as a dedicated but that G94 based 9600gt is at the edge as far as being good enough. Honestly I have done this and it can be a money pit on a new or middle aged setup and when re-using old cards for this purpose it has to be done right for it to really work. Fermi and Kepler cards have their weaknesses but for the power they do a good job and easy to track down cheap without being reduced to junk condition. The old stuff is showing the wear of use and time especially with poor driver support.