You really don't need that expensive a MB. Something in the $140 range will OC about as well I expect - I'm waiting to see the mid-range Z77 lineup. I'll say the Z77 version of all those boards was thoroughly overkill, and likely will continue to be so with much of the VRM moved on-die.
Unless you have a 2560x1600 screen or Surround/Eyefinity array, there isn't much point to anything above a 770 or 7970 - they'll both more than max out anything at 1080p.
You might want either a bigger SSD or a data drive to go with it.
PSU should be fine, but it's way overkill and overpriced for your build, unless you intend to SLI/CF, at which point it's not enough. 500W+ from the brands in my sig, 800W+ for dual GPU.
Pretty much. Tom's ran all their benchmarks in the recent 770 review at 2560x1440, and they were both usually near or over the 60FPS mark. With a 1080p screen, you only use a little over half as much processing power per frame.