GTX275 does not have DX11. It can run DX11 games in DX10 mode.
If you have a Nvidia-crack driver and an OS that allows multiple drivers (ie. W7) then yes, you can use a 5870 and GTX275 at the same time.
There really isn't any good games out there that uses physX. The only one I can think of is Batman.
and that's a game that's very specific to fans of that certain genre. Not really a mainstream game..