600W is more than enough for this setup - i think it will be around ~400W under load. I suggested a ~700W PSU because the efficiency typically is the best around 50-60% load, and because it should have at least 30A on the 12v rail for the geforce and for overclocking. But this should be only a problem if the PSU has two or more separated 12v rails. I don't know how this one is build, but newer PSUs have combined 12v power rails with 70-80A combined. So they are more flexible.
The other point with a server could be that maybe the fan is quite noisy because it is only optimized for power and not noise. But you'll see (hear).

ASRock should be fine, i'm running my farm on ASRocks (i7 970 and 2600K/2700K) - i don't know the Extreme 4. You should read some reviews for more info.

I'm no expert in GPU rendering, i'm only using CPU. But i think, mixing nvidia and amd would be no good idea, even if it seems to work. But maybe i'm wrong.

Okay, I managed to move entire array of groups however my project didn't save and because I did it after days of trial and error I can't now recreate my snippet but maybe someone knows how I can move that array of groups to another place (if I could get this first step again i should be able to get the rest). I'm pretty sure I used tranformation to get destination but can't figure it out.
Please help!!