850w is fine (dont know bout crossfire with those cards though)i like the plugs on the st -p power supplys the best (y the da cost more idk) they have 8/6pin so only one wire goes to each card the da has dual 6 pins (ugh)
i see a lot more rigs with the asus screans than the samsung well at least between the ones you posted.
the boards well the 5 and 7 are almost the same the 7 has water cooling or air with silent pipe you dont need either for a stock rig but the pipe comes free so might as well. the 7 has alternating power designs uses one set for a wile then uses the other set.
wether one is easyer / better to overclock on idk.
Trust me, 850 will cover crossfire on 5970's, and would be super overkill for only one, but if you are concerned about energy efficiency, PSU's tend to be more efficient in between to halfway and 80% of their maximum output.