The vast majority of games work just fine with 16gb RAM. I recommend running your higher speed modules as dual-channel and selling/repurposing your previous hardware.

Unless they are the same brand and specification, patchwork RAM tends to have compatibility issues leading to software hanging and potential crashes. YMMV.

ETA: Honestly you’re more bottlenecked by your CPU. I’d recommend finding a used Zen2 chip if you can, such as the Ryzen 5 3600. I have a 3600x in my home desktop with a basic Coolermaster dry tower. I’ve never gone over 70°F/~21°C on fairly high settings of TW:3K. That should speak enough of Zen2’s performance.

Ryzens, especially earlier gen are quite picky when it comes to using 4 RAM modules. Sometimes even RAM that seems to work in 4-module configuration causes memory errors, even if every module is good. I had a case like that where I was running 4x Corsair from QVL on an 1800X. The QVL only listed this RAM in a 2x configuration. Running it in 2x would produce no errors. Running it in 4x would produce errors. Whatever you decide to do, make sure you test it using Memtest86+ for 3-4 passes. In my case the errors would only show in the 2-4 pass. The first pass is a shorter and lighter one which allows for catching obviously defective RAM quickly.

First generation Ryzen struggles with higher memory speed. You very likely will not get the full 3600MT/s. There’s no real reason to buy slower, just be aware you’re likely going to hit a ceiling.

For gaming you might see bigger improvements from upgrading the CPU, maybe to an R5 3600. That and the memory are both going to offer big performance improvement.

Get the Corsair Vengeance 2× 8GB 3600 kit everyone gets, set the BIOS XMP to Auto so it uses top speeds, and done. More than enough for upper tier gaming.

Spend the rest on a new CPU.

Probably you will have to tinker with the ram settings inside your mobo to get it stable. IMO it comes down to you personally. Do you have a lot of programs running at the same time? Than go for the 24GB. But since it’s a planed upgrade you can benchmark your favourite games befor swapping, so you will see the difference yourself.

depends on if you are commonly cpu limited or not. If you are having performance drops from the cpu not keeping up then go with 16gb 3600Mhz to get a little more performance. If you never are being cpu limited then the extra ram would be better as it will be used for caching to help load times and reduce the chance of fps dips related to loading assets

For my applications, quantity is better. Since I do CAD work in addition to 3D scanning with only occasional gaming, I need the capacity.

While I am 3D scanning, I can use in upwards of 30GB of RAM (or more) in one session. CAD work may be just as intensive in the first stages of processing those files. However, I wouldn’t consider that “typical” use for someone.

For what you describe, I doubt you will see much of a performance hit unless you are benchmarking and being super picky about the scores. My immediate answer for you is quantity over speed, but you need to test and work with both configurations yourself.

I don’t think I saw anyone mention that under-clocked RAM may be unstable, in some circumstances. After you get the new setup booting with additional RAM, do some stress tests with Memtest86 and Prime95. If those are unstable, play with the memory clocks and timings a bit to find a stable zone. (Toying with memory speeds and timings can get complicated quick, btw. Learn what timings mean first before you adjust them as clock speed isn’t everything.)

Capacity > Speed any time. RAM speed is one of those things that only affects performance in benchmark charts. In reality, capacity is much more important.