A year range would work. Another way to sort them might be by bus type (1997-2005 would be *roughly* the parallel PCI/AGP generation, although technically the start point of it would be closer to 1994, and the last couple years of it start mixing in PCI-e slots, and have each of those bubbles encompass whatever CPU is lurking on that kind of motherboard.
... which, honestly, sounds like a decent compromise and would actually effectively kill one of the existing splits, IE, the "Pentium I" category. Just do this:
IBM PC/XT/AT and Clones (16 bit PCs, 8088 through 80286)
Early 32 bit PCs (386 through Pentium, ISA/VESA bus)*
Late 32 bit PCs (PCI/AGP/PCIe, pre-2006)
(* Most Pentium machines are PCI and the majority of 486s are ISA/VESA, but yes, there's gray area here.)
And it's covered in a reasonably coherent way until people start insisting that Core 2 Duos are "vintage".