A dedicated card is really only useful for the most part when you only have a single main card which would be only capable of running the game with decent frame rates without physX.
Having a overpowered SLI or TRI SLI setup and adding a PhysX card is pretty much pointless.
My 9800GT helped my single 470 in games like Mafia II, but in SLI it's the bottleneck for me.
There are other benefits to adding a cheap card that is capable of doing physX calculations. Running a couple extra monitors is one, also folding on it while playing games without a performance loss is another. I also can watch 1080p videos on another screen, while gaming with my SLI'ed cards and see my two 470's at 98% usage, while my 9800GT is doing the playback work on the other screen.