Since I'm building a computer by myself for the very first time, I'd like to hear opinions about some "open points" from more experienced people.

1) RAM - I'd like to have 16 GB of RAM. My motherboard has 4 DD3 memory slots. What's "better", to buy 4 4GB modules, or to buy 2 8GB modules? The 2x8GB modules are cheaper than 4 4GB modules, but what will it do with the system performance?

2) Hard disc. Right now I work with a 1TB WD Green drive. That's why I know, that I will never again buy a green drive :-) I was right now thinking that I will buy a small SSD for the operating system and either a 4TB WD Black drive, or 2x2TB WD Black drives and put them into a RAID 0. What is a better idea? The computer will be used mainly for editing video

3) Power supply - any tips for GOOD power supplies?

4) Operating system - is Windows 8 REALLY so horrible as I read and heard? I mean I worked with it just one day and I thought the ModernUI is a horrible piece of you know what, but people claim it has a better performance. Is it really worth to go to Win8 or should I stay with Win7 and wait till they release Windows 9? And no, Linux is not an option

Thanks