you have two expansion slots, but one is slower x4 instead of x16:
Which for the purpose that jcmcabee is asking about, won't make any difference as the HD 4850 will only be used for a third monitor. Not sure where SLI and Crossfire came from but neither of those are needed to run two cards for extra displays.
To answer the original question, yes it should work as Windows 7 allows the use of two different cards/drivers. If this was Vista then no, it wouldn't work.
You cant, Unless you use your GTX 560 as your PhysyX.
Actually you can, as above. The one thing that can throw a wrench into the works is Nvidia and Physx. Nvidia decided a long while ago that if you have a Nvidia card and AMD installed at the same time and try to play a game that supports GPU accelerated Physx, then that feature gets disabled at the driver level and falls back to software mode. There is (or was as I'm not sure if it is still being updated) a workaround to this by using a modded driver set that has had the restriction removed. It was/is mostly used by those with a fast AMD card and useing a slower Nvidia card for Physx.