soooo ... you have a GTX 260 that can handle PhysX on its own, and you're thinking of adding a second card for PhysX? Not so sure about that one.
Over on the nvidia forums, there was a whole thread full of people testing out the exact same thing you're talking about. The conclusion seemed to be that in most cases, if you have a decent nvidia card already, offloading PhysX to a second card provides zero improvement in gameplay, with the only noticeable effect being to use more power and generate more heat:http://forums.nvidia.com/index.php?showtopic=143047&st=0
One guy claimed a slight improvement, but on the other hand, a couple others claimed that offloading part of the work to an inferior card actually slowed things down.
With results like that, and considering that you'd be sacrificing bandwidth, it doesn't sound like this would be worth doing even if you got the second card for free, much less if you had to pay $100+ for it. I think the dedicated PhysX card option is mostly useful only if your main card is made by ATI.