yes, u can dedicate the gts250 for phys x, doesnt matter if its x8 or x16 anyway. but think about it, not all games support physX, and its not like u will miss a lot without it, but u will spend more electricity and bill. u better off selling the gts 250.
But the ones that do? I am sure that the OP would want to enable physx when available; it also takes a huge workload off the main gpu and the OP has more to gain by keeping the card rather then selling for little to nothing. Mafia 2 is supose to have physx btw; looking to be like a AAA tittle as well.
Batman AA really looks nice with the added physx features as well. If the game supports it; I wanna enable it