you wont get much of a boost. in my case my 9600gso i used for physics actually bottlenecked my 2 gtx260 in sli in ut3. nvidia needs to polish physx drivers.
Actually, Sli + discrete physics still isn't supported by driver. You have the physx option, but doesn't let you choose which gpu to run it on, as in single card plus another for physx. In other words, the "bottlenecking" is actually due to the PC trying to run physx off your gtx260.
Single card plus another for physx will always work.