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.
There are certain altered drivers out there that enable you to run a 9800GT in PhysX mode however I'd like to know your system specifications before I advise on this subject as you may not have the proper equipment to run a 9800GT in PhysX mode, and if so I'd recommend you sell your old card to a friend or on eBay etc.
I all ready said I do not know how to sell it online. The computer is custom built by me.
Intel i7 930
h50 water cooling
ati 5870 (xfx) rev2
coolermaster haf x
500 gb hard drive (dunno the manufacture)
msi big bang xpower motherboard
corsair 750w power supply