AMD produced at 120, while Intel's max was 100. It is mentioned in both of these articles.

http://en.wikipedia.org/wiki/Intel_80486
http://en.wikipedia.org/wiki/Am486

I'd rather have a high end 486 as it's better for DOS gaming. They're better for underclocking for slower games. The 485dx4/120 has an FSB of 40mhz with a multiplier of 3, while the P75 has an FSB of 50mhz and a multiplier of 1.5. The Pentiums are just faster CPU's in general, too.

The only two DOS games I can think of that actually need more than that would be Quake or Battlespire... with Battlespire recommending a P133. At that point, you might as well just go straight to something like a high end Pentium 2 400/450 with Win98SE, or maybe even a P3.

I think there are a few "optimal" computers for different gaming eras. For very, very old games, you'd want a 286. I personally don't play PC games older than around '88-90, unless it's some kind of major release or is really a decent game (old King's Quest or Space Quest games). Once games start needing the power of a 386, going with a high end 486 or a very low end P75 is the better option -- with my preference leaning toward a 486, as you now know. Beyond that, you get into Win9x territory, so you might as well go with a P2 or P3, and depending on if you plan on doing anything DOS related, decide on a PCI or ISA soundcard, but I think that's better left to a dedicated DOS computer (or DOSBox, even). Then there's Windows XP, which you could go as high end as you want since it's still supported by most new hardware.

Also, I purchased a GUS off eBay a few years ago but it turned out to be a dud. Everyone sells those things as untested because they either really aren't tested, or they just don't work and they want to make a few bucks and say they didn't know. I haven't tried my luck again, but would like to sometime in the future.