I don't really have any complaints in regards to any of the sdram based Socket 370 boards I have messed with, other then the Trigem Cognacs. And in regards to them its only a small gripe concerning disabling onboard audio and using pci audio cards, and the fact the boards dont have agp support. Thankfully the onboard audio on them isn't terrible, so you can get by using it if you find the one you got doesn't like your pci audio card, and I have not run into any problems running any given pci video card on them.

I have some Tyan 1854 boards that used the apollo peo 133 chipset. I NEVER could get a voodoo 3 PCI and AWE64 to co-exist on those boards. I tried everything, and I do mean everything, but could never get away from stuttering sound. I eventually put a PCI sound card in one, a Radeon video card in the other.

It is obvious that Intel P2-P3 architecture was generally better in the day, however, these platforms are not so interesting for retro purposes; especially if one wants to cover a decade... Most P2, P3 boards have one or two ISA slots only and do not underclock easily, as said before.

A K6-2+ or K6-III+ covers at least 1991-2000, the P3 is better suited to 1998- 2002 and that's it. However, anything after 1999 runs even better on a P4 and upwards... That is why I see no practical use for a P2/P3...

It's subjective. You might as well play DOS games and most Windows games on a modern machine combined with DOSBOX.

It is possible to build a BX machine that has a very wide range of support though. You can use an unlocked Deschutes or Klamath and run 133 MHz. Disable caches and you get down to 386 level. Or you can run a Tualatin at 1200-1500 MHz. 2-3 ISA slots is fine flexibility IMO.

Well, I was thinking about the original subject matter only (K6s vs Celeron), not including emulation.
I think your weapon of choice depends a lot on the cards and other HW you put in it as well; if one looks around here and queststudios threads, it is clear that when using REALLY old hardware (8bit ISA cards, first generation FM and LA synth boxes etc) it is often crucial to have FULL BIOS level ISA suport, including 8 and 16 bit I/O delays, exact clocks, non-shared ISA slots and so on...
I also have had far better experience with older boards, up to Socket 7 than any later models, that's all.