You would see a small boost in the PhysX games, yes. But not much. Of the PhysX games, I've played Batman: Arkham Asylum, Cryostasis, Metro 2033, Mirror's Edge, Dark Void. Most of these have very slight PhysX effects and you won't notice a difference. Batman and Cryostasis, however, can have fairly significant amounts of PhysX stuff going on at once. In these 2 games you would benefit the most from a dedicated card. However, in Batman you can easily play with no AA or 2xAA and get tremendous framerates to the point that even heavy PhysX wouldn't hurt FPS too much. It took 8xMSAA for Batman to bring my CF5850s to 60-65FPS. As for Cryostasis, well that game is extremely demanding on GPUs. Probably worse than Metro 2033 even, with all the heavy lighting and shading. I think a dedicated PPU would help quite a bit in this game as well.
That said, I don't think it's worth it. There's only around 11 PhysX games and of them only the 2 I've listed here are going to show much benefit. Most of the games were barely hitting even 11% usage of my GT 240. Batman go up to about 50% a few times, once I think it hit 80%, while Cryostasis is generally 30% or less. With SLI GTS 250s I can't see these PhysX spikes hurting performance too dramatically.
Hey guys I have a PNY GTS 250 running is SLI, would i benefit running a dedicated physX card???
Check out your game play with Fraps benchmark! nvidia's new drivers allow you to use one chipset as a ageia ppu proccessor. I Know Asus PHYSX released a new card pci-e x1 which is newer than the old ageia on pci slot! I would check out asus's website or online I think its 250-300! Also ask asus for their recommendation they have the fastest Phys-x card!