Not sure if my opinion would be popular here, but I just go with what I used to have at the time. So it is ok for me to have W98 on both a late 486 and a P2. Considering where I'm from, it was not uncommon to see quite a menagerie of systems during the same "era". This also applies to different components and peripherals. Basically, we had what we could get and make use of. I still remember using an old 9 pin dot matrix Citizen MSP-15E printer with a Celeron 300A. That was the age of all them bubble jets, mind you...
That said, I divide machines into "OS eras", and anything that could be reasonably expected to run that OS falls into that. Naturally, that produces a bit of an overlap in some cases. Also, I generally use Intel as a reference.
"DOS times" are anything up till early Pentiums,
W3.1 would be 386 and 486,
W95 would be 486,
W98 would be 486(late) to P3 (don't recall anyone using that on a P4)
W2K would be P2-P3 mostly,
XP would cover anything from P3 all the way to first Core Duos,
Vista for me is S775 P4 up till circa Q6600 heyday,
7 is pretty much Q6600 - present.
10 is... well, running it since Sandy Bridge (though tried before on Q6600, mobo's BIOS wasn't too happy)
Note that I've excluded ME and 8, as I've never had first hand experience of them. Seen 8 on a laptop, but didn't care enough.
Regarding video cards, there's an interesting pattern. Now that I think of it, most of the time GPUs were a few years younger than CPUs, as we'd keep upgrading those if/when possible, while at the time a significant CPU upgrade meant a new mobo also, along with new RAM... basically, something you couldn't afford to do frequently. So GPUs would be fresh, normally from "value" segment, and used for a long time also. I still had a PC with TNT2 M64 when GF FX series came out. And that PC was the same 300A Celeron.
EDIT: I also remember using a server 1 GHz Coppermine long into P4 era, it even got a GF6200 in its PCI slot for running Lineage II... and it did. To a point. At lowest possible resolution, of course!