From
https://www.tomshardware.com/reviews/best-ram,4057.html :
"Whether you’re building a new system or upgrading an older PC, the best RAM kit for your money depends on the platform you pick and the software you plan to run. The easy part comes from nearly universal adoption of DDR4 a few years ago. DDR3’s last gasp in the desktop world was Intel’s 6th-generation Core (Skylake) series and AMD’s Socket AM3+ and FM2+ interfaces. The move to AMD’s AM4 and Intel’s Kaby Lake platforms made DDR4-2133 compatible with nearly every recent motherboard (though you may occasionally find a DDR3 board hanging around in a bargain bin).
The hard part is understanding when faster RAM matters. If you’re running an Intel-based PC with an add-in graphics card, most programs won’t respond in a meaningful way to faster or slower system memory. A few will scale upward with data rate to the highest possible speeds, including some games and programs like the file compression program 7-Zip.
On the other hand, AMD’s current architecture is affected much more by memory speeds. The company’s “Infinity Fabric” (the internal bits that link various blocks of logic inside of Ryzen CPUs) is tied to the speed of the memory bus. You can read about this in detail here. So, increased memory speeds on Ryzen- and Threadripper-based platforms often translate to real-world performance gains. In games, that means faster frame rates at mainstream resolutions like 1080p, as well as smoother performance at higher resolutions.
Lastly, memory speed makes a big difference if you’re gaming on an integrated graphics engine using either Intel or AMD processors. Since the graphics silicon baked into CPUs doesn’t generally have its own dedicated memory (as discrete graphics cards do), upping the clock rate of your system memory also generally increases performance (though the ultimate speed-up varies greatly from game to game). So, you want faster RAM for those kinds of systems if mainstream gaming is important. Keep in mind, however, that if you have to pay top dollar for the fastest RAM to get playable framerates, you’re better off buying slower system memory and an add-in graphics card.
Confused yet? In short, you want faster RAM if you’re gaming without a dedicated graphics card, if you’re running an AMD Ryzen system, and in some edge cases with Intel chips. But if you don’t care so much about squeezing the best performance possible from your hardware, DDR4-2133 memory should be drop-in compatible with any modern PC platform."