I wanted to know how to (because i heard people are doing this already) have two cards like a gtx 280 and a 9800gt
and run the 9800 as a physx card and the 280 and a graphics card ?
I think (ie. never tried it) you go into the NVidia control panel, disable the Physx on the 280 and change some setting in the Multi GPU options panel to allocate the 9800 for Physx only. No SLI bridge is needed. I pretty sure you don't need any outside software for the operation, and I even heard on using an ATI card with an NVidia slave to cheat into Physix on cross fire. It has been awhile since I read the article.
You can still run ATi GPU + nV PPU on old drivers, which means DX11 and PhysX, but with the restrictions nV has put on the drivers, you wouldn't be able to use PhysX in any future title requiring the latest nV PhysX drivers.
The other thing though is it's only under XP (but no Dx10-11) and Win7 since Vista doesn't allow the drivers to work simultaneously. So dual booting may be an option for you. It's a hassle though, and to many too much of a hassle to care about PhysX.
The ideal combination in my opinion would be an HD5850 card + 8800GT as PhysX
in your case you can just use your GTX280 as PhysX and buy the newly released ATI card or wait for "Fermi" as they call it , it'll then do PhysX fine on one card
or you can wait for HD5870x2 and buy it for MAXIMUM-HARDCORED-GAMER-ON-EARTH
or even better wait ( last time I say wait, promise ) for "Hydra chip" only if you want a new mobo, that'll mix GeForce-GeForce, GeForce-Radeon or Radeon-Radeon (too good to be true I know) http://event.msi.com/mb/bigbang/
At first I thought, 'wow, they are reasonable'. Then i saw it was someone in the community that did it. kudos to the person that fixed it, but I'm not sure I'd want to rely on someone hacking the drivers for my PhysX calcs. If they had officially done it, I might have gone out and gotten one of their cards for fun. But if it is still in this sort of limbo, I think not. If this stays effective though...