1) Yes, I'm running an R9 270x with an i5-3570k
2) Power is sufficient, though if you plan to add an extra card, you'll need upgrading. I believe your PSU isn't modular, so wiring will be more of a hassle. If I were you, I'd put extra cabling in the lowest drive bay on your 912.
3) Don't shit out on cooling. Your case (and the gfx card's fan itself) will handle most of what the GPU gives off, and your CPU will last longer with better cooling. Preventative maintenance also helps with the longevity of your machine.
If you have any other questions, feel free to PM me. I have many of the parts you're looking to get.
600w is plenty of power for this current build, and good cooling will help with OCing your CPU.
edit: a new feature to the R9 series is that AMD has removed crossfire bridging, your motherboard takes care of that now
If you end up adding another GPU, go ahead (your motherboard has 2 x16 slots)
edit 2: I used to use AMD CPU's, but after some trouble I've had - never again.
regarding drivers: AMD Catalyst is easy enough to install, but I know people who have had trouble using either AMD or Nvidia software.