I have a PhysX card... As FastCargo said, a game needs to be written with the AGEIA SDK to take advantage of the PhysX chip. As an aside, a game that is written with the AGEIA SDK doesn't need the PhysX chip to run - it will simply use the CPU in its absence.
I think that a dedicated physics processor does have a place in flight sims, or any other kind of physics simulation. Althogh were are entering the era of multi-core CPUs, they still don't have as much processing power (for physics) as the PhysX chip.
Essentially what the PhysX chip does is handle collision detection and motion calculations. It also handles deformable meshes. Imagine a flight sim where you had true collision detection based on your plane model, and you don't have to do this:
CollisionPoint[001]=-1.40, 5.66,-0.91
CollisionPoint[002]= 1.40, 5.66,-0.91
CollisionPoint[003]= 1.23,-3.74,-1.17
CollisionPoint[004]=-1.23,-3.74,-1.17
CollisionPoint[005]= 0.00,-7.54, 0.50
CollisionPoint[006]= 1.21, 5.66, 0.53
CollisionPoint[007]=-1.21, 5.66, 0.53
Imagine that model deforming correctly when it gets hit by shrapnel; its wings flexing in a high-g turn or fluttering when you drop bombs; tails buffeting in high-alpha...
I think that if a sim were to be written with the AGEIA card in mind it could be truely awesome.
Dreams aside, until there are more games that use AGEIA, I would also recommend holding off on it - but if nobody buys it, who will program for it? ;) Catch 22.