I've been having this dilemma too - Which is the most suitable Classic machine you can build?
I've been experimenting with various mobos and cpus from 486 to Pentium 1s with different speeds from DX33 to 166MHz.
IMHO, there are 3 factors to tackle:
1.) Gameplay Speed : e.g. Wing Commander I. This game requires the "sweet spot" to play it perfectly. I think Amigaz has posted his assessment based on various speeds of cpus for this game.
But if you're gonna build a system to play this game, then it would make playing later games useless in this machine - too slow.
There are also the last generation of DOS games which are a bit slow to run in Dosbox. This will have an impact on the smoothness of the gameplay and in turn can hamper the real experience. These kind of games require a fast DOS machine. The fastest you can build.
2) Graphics speed : e.g. Bethesda Softworks' Terminator : Skynet, Duke Nukem 3D etc.
- These games have support for 640x480 SVGA and higher resolution modes. The same holds true for some other sims like U.S. Navy Fighters and so on. To play these games, you have to get the fastest graphics card that can push up the frame rates in pure dos. Those days, you need to have a fast cpus to get good frame rates too. Building this type of machine will also render many old games unplayable - too fast.
3.) Hardware factors. Here we have some games which insist on running on pure DOS only and which require 100% SB compatibility and VESA support. These games will run best in pure DOS. Most games of this category also have a common requirement of fast CPUs/Graphics Cards.
I think the best thing to do, IMHO, is
1)Build a reasonably fast system, just enough to play the last generation of DOS games. I'm having a Pentium 1 based system with CPUs from 75MHz to 166MHz MMX. Currently I'm using a 133MHz Pentium 1. Put in a S3 based PCI card and pass it through a Voodoo or Voodoo 2 card. I'm having an S3 Virge based card which passes through the Voodoo2 card.
(Descent II supports ViRGE, and some Win95 games have support for ViRGE too.)
I have 2 hard drives in it - one for Pure DOS and the other for Win95.
Also I think that a later generation mobos (circa Pentiums) will have a better flexibility to tweak in the BIOS - like choosing which drive you want to boot first, and so on.
2) For a speed sensitive game which runs just too fast, it's much more convenient to play it under Dosbox. In Dosbox, you can fine tune to the actual speed you want for each game. Games like Wing Commander I will be immensely benefited by this. And you get to play newer dos games unhindered in the classic DOS machine.
Two computers - one latest system with Dosbox and another DOS/Win95 based machine PI to PII for the later Dos games will have a good flexibility.
I've even tried with a PIII 450MHZ system with good outcomes for newer Dos games.
Of course these are from my experience and the factors can be extremely different with many other different hardware combinations and the speed factor tolerability maybe individualized.