1. Not really. Just plug the HDMI cord into the GPU and TV. You may need to adjust for overscan.
3. In-game, both will work. Many games have 360 controller support. On the desktop, obviously keyboard and mouse. However, you can set up XBMC or some other front-end, or use Steam Big Picture Mode.
4. No, Apple TV is a streamer.
5. Yes, a PC is a PC. My HTPC runs the same Windows 8 as everyone else.