If you're not going to get another 580, the PSU and mobo are both overkill. If you are going to get one, then this is good.
Skip the Noctua. You don't need it. Get a Mugen 3 and save $50.
If you've got the external, consider whether you need an HDD at all. Maybe just get a backup drive instead if you don't have one already.
1) Considering the current high prices of hard drives, why not just defer the hard drive to later? A 120gb ssd should hold the os and 6-10 games.
2) A GTX580 needs only a quality 600w psu. If you are planning on sli later, then ok on 900w. My plan for upgrading later would be to look for a single strong kepler card.
and a 750w unit would be ok for that.
3) The Antec 1200 is a fine case, but rather large. Do you really need all that room?