I tested an 8800 GTX and a 465 GTX pared with my 670 Amp!
I ran numerous benchmarks with 3D Mark Vantage (It has a dedicated PhysX score) and the MSI Afterburner app.
I ran a few games, like Batman AC and Metro: Last Light.
My conclusion is this:
Anything less than the 465 will reduce performance. The 465 gives the slightly better performance as the single card, but not enough to justify.
Dedicated PhysX cards are more or less a waste. You are better off just getting another 660ti and running SLI. You would basically need a 650 to have any noticeable difference in the games that do use it, and it won't increase your overall FPS, it will just maintain a smoother FPS.
The 8800 could not run the PhysX as quickly as the 670 could run...