NVIDIA, there is no question about it. The 1070 and 1080 just annihilate anything AMD has put on the market. I would not recommend going for a multi-card configuration since it tends to increase frame time and latency which can be really annoying and make the high framerates you gain redundant.
Intel absolutely destroys AMD as well when it comes to CPU performance. A quad core intel is outperforming a octa core AMD. For a typical gaming PC and some video/CAD work the i7 6700K should be fine instead of opting for the more expensive, more cores of 2011v3. Really depends on how much horsepower you are willing to pay for.
Air cooling. The Noctua NH-D15 is quieter and performs just as well as any AIO cooler (pump noise anyone?). Custom watercooling only if you are willing to spend an ungodly time on maintenance. Been there done that, would not recommend for most people.
Motherboard sockets are just about CPU compatability. 1151 is for current generation consumer CPUs and 2011v3 is for enthusiast/workstation grade CPUs (more expensive, more cores). Just make sure you get a Z170 chipset, and not H or B chipset if you want full features and overclocking.
Unless you are going for a 2011v3 build, 2x16GB will perform just as well since you are limited to dual channel, and it gives you the ability to go for 64GB in the future if you deem such overkill necessary. The key to selecting RAM is balancing timings vs frequency. 2400MHz RAM on CL14-15 is better than 3000MHz on CL17-18.
As for power supplies, make sure you get a good brand. Corsair AX/HX series are very good, also EVGA SuperNOVA P2. Stay away from Corsair's RM/CS series, they are cheap garbage. 600-700W is plenty power for a single card. 80+ Platinum is recommended by me.