Hardware is very much a moving target given how often new products come out and old products are repriced. For example, the 16 core / 32 thread AMD Ryzen 9 3950X is now out and available for around $795, and Gen 3 Threadrippers are out with the 24 core 3960x at $1390 and 32 core 3970x at $1950. The 64 core / 128 thread 3990x will be $3990.
The other issue is how long a given buy will be supported by either motherboard vendors or drivers, in the case of NVIDIA GPUs.
In terms of basic performance, there is good advice in the Performance Tips topic and in the GPGPU topic, both of which should be read carefully by anyone thinking about spending big on GPU cards. Takeaways from those topics:
1. GPU does nothing for tasks that are not sufficiently computationally demanding to be worth dispatching to GPU.
2. Competition for GPU parallelism is CPU parallelism. Having lots of CPU cores means even fewer jobs will be worth dispatching to GPU. So in general purpose GIS work, the new wave of inexpensive manycore CPUs means there's even less reason to overspend on GPU.
3. Don't overspend on GPU while spending less on CPU, SSD and main memory. For most people, there's probably no point in spending more than $500 on a GPU card. If you are into gaming, then of course spend what you think is cool for gaming. It will almost certainly be way more powerful than you'll ever come close to utilizing in GIS. As the Performance Tips topic puts it, "multi-thousand dollar GPU cards are far past the point of diminishing returns."
There's no one-size-fits-all rule that says "this is what you should buy" because it all boils down to the many variations in exactly what you are doing. Doing a lot of work involving no computation but lots of shuffling from file to file? Better to invest into faster SSD. Doing a mix of general purpose work in a Windows system that has the usual mess of other stuff running in background? Get plenty of RAM, a good SSD and a manycore CPU. Spending time doing exceptionally mathematically complex computations on rasters? Probably worth it to spend $500 instead of $100 on your GPU.
For most people, there is not much point in overspending on CPU, either. As reported in other threads in this forum, Manifold is now in the process of tuning for huge thread counts on the desktop. Today, there's not much point in spending $4000 to get 128 threads, because those likely won't be all that much faster than, say, 32 or 48 threads. That will certainly improve in the months ahead but still, there are lots of other parameters, like having enough memory bandwidth for 128 seriously active threads to be able to work with memory. For most of us, what we do on a daily basis doesn't involve the sorts of tasks where it's useful for 128 threads to go to work given the architecture of everything else that is going on.
If you have endless money than sure, just buy the best and even if you rarely use all the capability you'll know it's there.
Manifold's approach, though, is mainly aimed at trying to help users get the most out of a limited budget, a bang for the buck approach. While none of this is precise, and this all will change probably tomorrow as prices and products change, my own best guess for "sweet spots" for new machines would be:
1. Professional workstation: AMD 3900X (12 core / 24 thread) with 1 TB or 2 TB M.2 Gen 4 PCIe SSD in the motherboard, 64 GB RAM, 3 monitors and $200 to $350 spent on the GPU card.
2. "Hot" workstation: AMD 3950x (16 core / 32 thread) or Threadripper 3960x (24 core / 48 thread), with dual 2 TB M.2 Gen 4 PCIe SSDs in a RAID configuration (if the motherboard permits), 128 GB RAM, 3 monitors and around $500 spent on the GPU card.
I'd avoid prior generation Threadrippers because the motherboards for those don't support Gen 4 PCIe M.2 SSDs, where you get twice the speed of Gen 3 SSDs with much lower costs. As always, "data access is the ball and chain of parallel processing" (Rotow's Law).
In terms of upgrading existing systems, the following is a useful set of priorities in which to spend/upgrade:
1. Use 64 bit Windows, if currently stuck at 32 bits.
2. Install at least 16 GB RAM. 32 GB is better.
3. Install an SSD. If you don't have an M.2 slot on your motherboard, at least get an SATA SSD.
4. A GPU card, at least a cheapo $100 or $150 card, if you don't have one.
5. Upgrade the CPU if you can plug a CPU with more cores into your motherboard.
When buying CPUs, it's better to spend money on more cores, more RAM and faster SSD than it is to spend disproportionately more on CPUs with higher clock speeds but fewer cores. If you are rich, than sure, a faster CPU clock with faster memory is fun to have, but most of the time you won't notice any difference.