I have never figured out what the PC numbers mean (PC 25600 or PC 19200 as an example) and really have never figured out what memory is the "right" memory for a motherboard/CPU combo. Can anyone explain it a little for me?

As an example, the CPU I'm looking at right now says "Supported memory: DDR 4 2666. Now I'm assuming that doesn't mean your one and only choice is 2666 speed, but does this mean that other speeds won't be as... I dunno... efficient? And then the motherboard, whichever one you choose, is rated for certain memory speeds to. So do you look for a MB that not only matches the chipset of the CPU but also the memory speed as in that makes the two work together more.... again only word I can think of is efficiently??

Guess I would just like some overall advice on memory. Up to this point I've always ensured I got the correct type of memory (DDR 3/4 whatever) and then basically just chose brand and speed on my own. But wondering if maybe it's better to try and match things better with your MB/CPU combo... again I'm talking speeds only... I know you have to match the correct TYPE of memory.

Maybe this gives you at least additional information
http://forums.crucial.com/t5/tkb/articleprintpage/tkb-id/dram%40tkb/article-id/91

I cannot help you much with the rest. To be honest I did not even know that CPUs would only support specific memorys(speed. Whenever I built my PC I just read the mainboard manual and tried the fastest speedthe mainboard supported.

The speed of the RAM module is the highest speed the module was tested at and confirmed to be working and stable.
You can run it at slower speeds - at worst the base JEDEC configuration.
It may or not be faster, but given that many manufacturers bin their modules and price them accordingly it probably won't run much faster without errors. On the other hand due to RAM shortages identical kits are sold at different price brackets too - so what you end up with is pot luck atm. Paying more guarantees you that the module will run at the advertised speed; if you don't want to pay for that, it's down to silicon lottery.

The CPU supported speed is the highest speed the memory controller on the CPU supports - anything above that is considered overclocking and may or may not work.
If you have a highly binned cpu chances are it will work just fine with faster memory, in fact if you have a Ryzen or Threadripper you will actually need to have faster memory, typically 3.2GHz is what you want for those.

Finally, the routes on the motherboard must be able to support the high frequency memory access as well; so you have to make sure the motherboard supports the selected speed as well.

Your RAM module will only run stable (or at all!) at the highest commonly supported speed of the 3 components listed above.

You should also buy matching modules if you plan to use dual-channel (common) or quad-channel (not so common yet) memory configurations, ideally sold together, to make sure the modules are really identical - there are timings not listed in the RAM specifications, that can make your life difficult.

AFAIR 8 MB is essential for games Duke 3D or Doom II.
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Oh wait...:)

Aye, it the annoying thing is you need a math co-processor. My 386sx doesn't!

YES! That was very very helpful. THANK YOU. Wish I had read that years ago LOL. Now at least the numbers next the memory modules on sale make a lot more sense to me. Now to just match them up right with "my" motherboard. (LOL, I'm _thinking_ of building another rig... the last rig I'll ever need/buy/build)

386DX also have no math co-processor embedded.
I guess you want to say 486SX? :-P

Nope, my 386sx didn't have a math co-processor, my friends 486dx did. Wasn't try to do a run of all the systems available at the time.

Neither does my 68LC040. I really want a full 68040 machine.

Ha, just searched eBay.co.uk for 68040 and it came back with a processor slot - as you would expect - and the second result was a kitsch light blue whistling kettle!
https://m.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2056088.m570.l1313.TR1.TRC0.A0.H0.X68040.TRS0&_nkw=68040

Irony?

Right, that should probably be MC68LC040 and MC68040. I could, of course, upgrade my Performa 475 (it already has 26 times the storage of the original configuration and 9 times the RAM), but honestly, I'd prefer getting a machine from the Quadra line which was originally built to be powerhouses.

Nothing that a 80387 couln't fix.
Go for the monster 486DX2, then sell your car to add a Pentium Overdrive on that baby. All the options to enhance your computing experience...

Ahhhhhhh,now all is explained.