You won't be able to get 256gb of RAM on a consumer board, you'd have to go server for that. And even then 256gb of RAM would cost around $5,000 assuming it's actually good RAM. Are you sure you are looking in that kind of price range?
The X is benching better than the Z, and it's cheaper and newer. The actual letter Z being after X in the alphabet doesn't matter :P
If you go a dual PC build then usually one will be your main computer and the other only for recording, you wouldn't need to make two equally powerful because then your main computer will be underpowered and your secondary will be overpowered. It's more efficient to spend as little as you can on your secondary and make your main as powerful as you can.
SATA = Serial ATA, it's the interface that most harddrives and SSDs use, it's not as fast as PCI (doesn't have as high bandwidth and a few other reasons).
SLI = using multiple graphics cards, so instead of 1 titan X you could have 2 or 3.
SSD = solid state drive, it's like a harddrive but made out of components more similar to RAM or a thumb drive, there's no moving parts. They are much much faster than harddrives.
DDR4 = the newest kind of RAM to become available on the market, it's faster and has more cutting edge features basically.
Benchmarking = testing to see how good components work in the real world, that's the way we compare two components eg CPUs or graphics cards to each other. Numbers are all well and good but because there are many differences between different components it's not always obvious which one will perform better and in what situations. For example sometimes a slower CPU with less cores will outperform a faster CPU with more cores, basically because it's more efficient so even if it runs at half the speed if it's three times as efficient it will still be faster in practice.
I doubt you would have any problems with a fractal PSU, but usually you would buy one from a company who specialises in PSUs. I posted a list earlier;
http://i.imgur.com/tgrbCnr.jpg
We don't actually know how much wattage you need, so you don't need to pick one yet. Also there's efficiency ratings that go from "80+" (the lowest rank), "80+ bronze" all the way up through silver, gold, platinum up to "80+ titanium". This is a measure of the efficiency, but it's a general indicator of the quality too - of course it's no substitute for real world inspection and testing which is why we also consult a list like the above to find a suitable PSU.
Is there any kind of ballpark or upper limit to your spending? Would you literally be able to buy a $100,000 computer? Would you even want to?
I'm ok with explaining anything you want to know and helping with choices, but it's impossible really to make any good suggestion without knowing what kind of price range.
For comparison, a usual good computer would cost about $1000, and for an amazing awesome computer $2000. And like I said before, there's absolutely no roof, you can spend $10,000, $100,000 or $1,000,000 and still have room to upgrade... Not to mention new components come out yearly that you can buy to upgrade further.