1- Is it worth it using another video card for PhysX operations only?
2- If yes, which low-power/hopefully silent card do you suggest me to use for this?
If you enable PhysX, you do need a dedicated card, with modern games, as PhysX requires high GFlops & high Texel rate, so lots of CUDA cores, preferably the one with computing power, and lots of MHz: any modern graphics is hit by it, even a mighty GTX 680, GTX 770, and so no...
But then a good PhysX card with modern games it could be at least as fast as the GTX280: does it worth?
A long while ago, along with a GTX 275, I used a 9600 GSO 384Mb as a PhysX card, and with the games of the time it was good.
Today I don't know whether it's enough or not (however, gaming resolution will also matter).
If I were in the market for a quiet PhysX card, more probably that not I won't buy anything less than a passive GT640, but probably I would also look to a used GTX460 1Gb, like a Gigabyte OC (a GTX 560Ti could be surely better, but I don't know any reasonably quiet).