AMD's new 64-core/128-thread 2900 MHz Ryzen Threadripper 3990X is the new beast on the block. The core count is a world's first for a HEDT (High-End Desktop) processor, and it is squarely aimed at the high-end 3D rendering, visual effects, and video professionals.
But such silicon doesn't come cheap -- a single chip comes in at $3,990 (double the cost of the 32-core Threadripper 3970X), or just over $30 a thread.
But that's just the beginning of the spending spree you need to go on to support this behemoth.
First is a motherboard.
While the new sTR4 socket has the same number of pins as earlier-generation SP3 socket, the pinout is different, so the Threadripper 3990X needs a new TRX40 motherboard, along the lines of the ASRock TRX40 Taichi or GIGABYTE TRX40 Aorus Xtreme. These motherboards vary in price from about $450 to $850, depending on what features you want, but all support the PCIe 4.0 standard, up to 72 PCH PCIe lanes, faster RAM (up to DDR4 3200), and loads more USB and SATA ports.
On a plus note, if you put a Threadripper 3990X into an older SP3 socket, nothing bad will happen, it's just that the system won't boot up.
On the RAM front, the TRX40 motherboards for the Threadripper 3990X support up to 256GB of RAM, which sounds like a lot, but on high-end systems really isn't and works out at only 2GB per thread.
Another issue that you are going to run up against is that Windows 10 Home or Pro can't handle 128-threads without there being quite a hefty performance hit (as measured by AnandTech). The solution, predictably by this point, is to spend more money. You can choose to hand Microsoft $120 to upgrade Windows 10 Pro to Windows 10 Pro for Workstation or grab a yearly Windows 10 Enterprise subscription for about $85.
Alternatively, you could run Linux on your Threadripper 3990X system.
It's not a question of how fast you want the system to go, but how fast you want to spend money.