the psu looks fine to me but i would get 8gb instead of 4gb RAM
also you might take a look at the R9 270 (non X) i have a MSI R9 270 Gaming OC 2gb (1050/1500 core/mem instead of 955/1400) it perform almost same as a X version once overclocked(ofc you can OC more a 270X but the price difference between the 2 make my solution quite viable) and only a 6pin PciE needed, and they are way enough for gaming at HD resolutions
i have a 600w psu but ~500w could be enough too a 850w would be overkill, the thing is most of the manufacturer exagerate the PSU recommendation
i had a SLI of GTX580 Matrix (2x8pin) along with a i7-920 @4.4ghz (125w on stock clock 240w oc estimation ) on a Rampage III Extreme with a 700w 80+ gold psu (Asus recommended 700w for one 580 Matrix alone) but i never ran into any instability, no matter what i did with the rig.
the 750K +
A88X is also viable for cpu mobo
6300 setup with a 270X assuming 4 120mm fan for the case
Minimum PSU Wattage:326 W
Recommended
PSU Wattage: *
6300 setup with a 270 same fan pattern
Minimum PSU Wattage:315 W
Recommended
PSU Wattage: *
750k same setup 1
Minimum PSU Wattage:330 W
Recommended
PSU Wattage: *
750k setup2
Minimum PSU Wattage:319 W
Recommended
PSU Wattage: *
my actual build
Minimum PSU Wattage:311 W
Recommended
PSU Wattage: *
ofc
http://extreme.outervision.com/PSUEngine give a min and recommended but you should always go a bit above but it can serve as a reference
for instance the i7 SLI setup was
Minimum PSU Wattage:710 W
Recommended
PSU Wattage: *