Ryzen loves extra ram! Look for b-die/e-die ram, you wont regret it! Or so I read before upgrading my PC. At the end of the day, I hesitated and lost the opportunity to buy that 100 dollar ram upgrade anyway because it was sold out and ram was in high demand due to the new CPU launch, and I'm glad I did . Would that extra 100 bucks be better off spent on a better graphics card or processor? It seems so.
https://www.techpowerup.com/review/amd-zen-2-memory-performance-scaling-benchmark/2.html
I just play open world games at 1080p and occasionally watch movies at the same res. Don't care for much else.
If you want to run 1440p monitors or 144hz monitors, a better GPU is necessary. 1440p 144hz makes you never want to go back to normal 1080p 60hz monitors. My 1070 can't support two 144hz monitors unfortunately, it only has 1 display port and HDMI is only 60hz.
The NVMe's are a great help for general OS usage and game loading times. I have a Samsung 970 EVO at home and work. The 500gb version is almost exactly $100
Frequently overlooked is the performance of the North Bridge.
https://en.wikipedia.org/wiki/Northbridge_(computing)
A good north bridge means your core parts (CPU, motherboard) and I suppose RAM are getting the best possible pipelines.
It depends on the use case. Also on the upgrade, is it more ram, faster ram, ECC. Often faster storage helps more for feeled performance, but if you have enough ram and use the same data all the time ram speed matters as well.
I have been a game engine developer for consoles since the PS2 days. The consoles are generally shitty stripped down hardware where one stick of low latency RAM at the time costs more than the entire console. RAM never ever was a bottleneck, and my specialty was allocators and bandwidth between the CPUs, GPUs and co-processors like FPUs, SPUs or VUs. That cheap shit RAM never once limited me. The problems were always in the memory controller on the motherboard, which decides which busses every memory access uses. Lower latency RAM can help hide this, but the mobo, CPU and GPU limit the RAM usage. Solve those bottlenecks first, then get lower latency RAM.
I've fallen into the trap many times of buying low latency RAM. It's not worth it.
What do you use your desktops for? If you're say compiling large codebases, then better CPU is great, otherwise not really necessary. Better GPUs can be good if you play games, but many games are really often CPU-bound.
IMO, the two most useful places to dump $100 is a faster SSD like a NVMe for your OS to be installed on, or a higher quality motherboard. Some lower quality motherboards can't serve the bandwidth USB-C/thunderbolt advertise because they will starve the connection for hard drive access.