Depends on what titles you plan to run.
In titles that actually use PhysX (Borderlands 2, Killing Floor 2 (This uses Nvidia FLEX)) it might be useful.
The two main issues with PhysX cards are not having a use for them (not many titles run PhysX anymore) and speeds.
If the PhysX card is running too slowly the main card will be waiting on the computations, resulting in worse performance.
The 670 might be fast enough, but it depends.
Nothing wrong with trying it out.