Hi. I was summoned here by Hive account@free999enigma to aid you in picking out parts for a new PC. I'm not sure what kind of budget you have available, but this is going to cost you over 1K$ clearly. I can't give you exact prices, on account of I don't know what region you're from and you can always find promotions going on for some parts.

Let's get started with the most important bit, the CPU. If you're planning on getting the PC before April, I would suggest going for the Intel i7 8700K. With 6 cores and 12 threads that can auto-clock to a very high frequency you will have a machine that can crush games and streaming with no worries. You'll be able to do encoding and use just about any other application at the same time. Even some games that aren't very demanding. If you are going to buy the PC in the spring, I would suggest taking a look at the Ryzen Refresh series from AMD. Never touch Threadripper unless you're planning on making a workstation, those are not general use CPUs.
This CPU does not come with a cooler, so you need to get one separately. There's a lot of options here, you could go fore a closed loop water cooler, like the Corsair H110, but that tends to cost around 150$, which is a lot. You can also go for something like the Noctua NH-D15 or the Thermalright Le Grand Macho

For the motherboard, Asus is a fine choice. I would go for the ROG Maximus X Hero if you want the absolute top of the line part. With this motherboard you have an overclock profile that will take the CPU to 5GHz automatically. At that point, there really isn't anything you won't be able to do with it for the next several years. The motherboard has plenty of space to expand and all the connectivity you'd need for the future.

In terms of RAM, good GOD do not buy RAM now. Everything costs 2x what it used to and it will continue to increase in price. It is a horrible time to buy RAM now, but if you must, go with 16GB of anything you can afford.

SSD: I suggest you get a Samsung 960 Evo of 250GB for boot and some games/software. There are versions with more storage space, but they are a bit expensive. The advantage of the 960 series is that they reach speeds that will make any loading almost instant.

For the GPU. There isn't anything with more than 12GB VRAM on the market and there won't be for at least a year. If you can afford a 1080 Ti, sure, go for it, it is the current best video card on the market aimed at the general public. Nvidia is starting to frown upon SLI, so you will start seeing less and less advantage with having two video cards installed, unless the game developers start implementing Explicit Multi-Adapter mode in games, which will connect multiple graphics cards by default. If you can't afford the 1080ti, go for the 1070 Ti, it is cheaper than the standard 1080 and almost as powerful.

For the case you can get just about anything you like, they all tend to be great these days. A personal favorite of mine is the Corsair 400r, which usually costs around 120$.

As for the screens, you don't need them both to run at 144Hz, since you'll be using one to monitor your stream and the other one for playing. It would be cheaper to get a good 144Hz screen and a cheaper 60Hz one. I'm saying this because a 27 inch 144Hz screen is very expensive, it is the second most expensive part after the GPU. But if you're going to go through with it, get an Acer Gaming Predator XB1 XB271HU 27. It is 2560x1440 resolution, so it can fully utilize the GPU. It costs around 800$. If that is too much, get the Acer Predator Z271bmiphzx , it is a lot cheaper. Both of these have G-sync, which is very useful with the Nvidia card at high refresh rates. If you do not need G-Sync, then get something like the Acer Gaming GN276HL. It is a lot cheaper and only 1080p, sadly. Also, because it is a TN panel, it will be very fast, but the colors are not great. You could also try the AOC Gaming AGON AG271QX or the BenQ Gaming EX3200R.

As for the PSU, get anything made by Seasonic with around 700W or 750W. That should last you a long time, even with two graphics cards, if you want to upgrade in the future.

Again, I'm not sure what budget you have at your disposal or when you want to get the computer, but if you're looking for top of the line performance, this is your current best choice. Let me know if you have a smaller budget in mind and I'll see if I can find something that will match it.