If space is an issue, a large ssd may be a bit pricey. One way you could maintain the space you need and still get near ssd speeds would be to get a Z68 chipset mobo so you can add a small ssd later for intel smart response. If ssd like performance is not an issue, the one thing I would recommend from you list is to maybe up the wattage of the PSU, so you're a bit more free to add a 2nd 6970 later if you want for X-fire. I went with a gigabyte Z68 board, and a 1.5TB hitachi drive with a 20gb intel SSD for smart response, and I'm very happy. I also went with the 850 watt corsair, and run 2 OC'd 6950's off of it. I was happy I gave myself some room with the PSU.
I really could not care that much about OS loading speeds, as long as everything is ready to go under 30secs. I think probably space wont be such a big issue because the only thing i will have on my wd caviar black is games and some random applications. I still have a Hitachi 500GB which i am probably going to install Hackint0sh on.
I heard that when CFing with a P67 the performance is bad on the 2nd slot because it runs at x4 not at x8 like the primary slot?
What do you think of the Corsair TX-850W? The place where i get my computer parts from only sells up to 750W for the Corsair HX series so getting a 850W HX series is not a option.