1. I believe you just have to use older drivers (some newer ones prohibit a main ati card + physx on another card), but I'm not sure which ones.
2. What games do you play? For the most part it won't matter. If I were you I'd just sell it and save up for something else.
3. Physx doesn't even apply in most games, and in the games that it does work in, it's usually not a big deal. It can add some nice effects though.