Crossfire / multi generation & other noob questions

OK, I'm drooling over the wonderful new stuff available today, getting close to pulling the trigger, but I have a few questions. I'm relatively certain about cpu, motherboard, and a few other things.

Intel i5/750 (150 at microcenter)
Asrock P55 extreme (139 newegg)
500-640GB HD (I'll see what deals there are coming up, looking at things like WD Blue series)

I'm all over the place on video cards though. So, here's a few thoughts and questions, any advice or feedback is appreciated.

1) Can you crossfire across generations? If I get a 4850 today, then a 5xxx in the future, will it even work? Would it support dx11?

2) I'm very torn between generations. 4850, can probably get for around 100, great performance, etc. I'm a little worried about heat from this card, vs. the 5750 which seems to run much cooler, but the 4850 seems to win on performance, and definitely on price (and availability).

3) Then every so often I look across the aisle at the Nvidia cards, and wonder how cool PhysX really is.

Games - X Plane, MS Flight Sim (current or future versions). Want to be able to run other of today's games reasonably well (which ones? Whatever moves me at the time). But this PC is used much more for business productivity than 3d gaming, so power/heat is a bit of an issue.

Thanks!