I recommend getting a good USB drive and keeping it attached at all times with a bootloader. Saves space on drives and helps reinstalls / troubleshoots. I hooked up several of these HDDs, and two of these SSDs. I also added a USB headphone amp (DAC): this is the one I use that works great in Mac OS X, with this pair of headphones.
Whatever is affordable and compatible at the time. Haven’t had bad experiences with Corsair.
The old Mac Pro tower had a 1000W supply; lots of headroom, very efficient, very quiet.
Whisper quiet, very accesible and easy to build in, and — dare I say? — beautiful to look at.
Powerful, yet affordable CPUs that are excellent at both multi-threaded and single threaded work.
Beautiful, highly compatible motherboard with Thunderbolt expansion option (!).
Extremely good performance / price ratio, very high performance, low power use and heat.
Recommended: Same or simiilar. Whatever.
Recommended: 600 watt at the very least.
Recommended: here’s a whole list of good cases.
Recommended: Same or dual GTX780.