It really depends what you want to do with them. I for one am a greedy bastard and I like to play everything in as high detail and on silly resolutions - as such, I tend to go slightly over-spec with my builds. Here are some of my machines and their purpose:
Main retro rig: AMD K6-III @ 550Mhz / Aopen AX59PRO (VIA MVP3 ATX) / Riva TNT2 PRO / 2x Voodoo 2 8MB SLi / Yamaha YMF744 / Primax Soundstorm M16C (GUS GF1 clone)
- I use this machine for most of my retro needs. The CPU can be clocked up to 600Mhz and as low as 200Mhz while disabling all cache using a utility called SetMUL. This lets me play speed sensitive games in DOS. At 200Mhz with all cache off, it's about as slow as a 33Mhz 386.
- The Yamaha card is used for Windows games as well as older 3dfx DOS titles with MIDI music, like Descent 2. It has an excellent MIDI driver closely emulating a Yamaha SW60 synth. As such I run as many games under windows as I can - and most DOS games that support General MIDI work well in a Win98 DOSBOX.
- The Gravis card I use in pure DOS. Since it's not PnP, it doesn't push it'self on the windows environment allowing for the proper operation of the Yamaha card. In dos however, the Yamaha does not have a General MIDI TSR, forcing me to use the GUS. It's great for late DOS games, especially ones that have native GUS support.
- I mainly play stuff like Red Alert, C&C95, Dungeon Keeper, Descent 2 (with 3dfx patch), Carmageddon (3dfx patch), Duke Nukem 3D, Nuclear Strike, GL-Quake, Quake2, Hexen II, Expandable, Populous 3, SHOGO, Mechwarrior 3, Pandemonium, and others elusively on this machine.
Secondary retro rig: AMD Athlon XP 3200+ 2333Mhz (166x14) / Abit AN7 (nForce 2 Ultra) / 1GB DDR400 / Soltek FX5900 XT / Sound Blaster LIVE!
- This machine dual boots win98 and XP. XP is mainly for maintenance, networking and internet access since not all my favorite games run well on XP but do so on 98. I also use it to put stuff I get off the net on floppy drives for old machines with no network access. This machine is fast enough to run all my games at 1280x1024 with some AA.
- I run Quake 3 Arena, Unreal Tournament 99 and 2003, Dungeon Keeper 2, Black and White, Descent 3, Descent Freespace, Mechwarrior 4, Legacy of Kain Soul Reaver 1, 2 and Defiance, Sacrifice, Unreal 2 and many others exclusively on this build.
I have a couple of 486 (586) builds, but they're more of a niche since they're too slow for some games and too fast for others. One houses my Yamaha SW20-PC since it won't run on faster computers, but I'm planning to move it to a Pentium 1 build. I use that 486 for sierra games (King's Quest, Space Quest, Gobliins, etc) since they all support general midi and the SW20 sounds best out of all GM midi cards / modules I own. The 386 and 286 machines in my signature are for speed sensitive games. The 286 can run XT games very well.
As for your machines, you really didn't post too many details. If you want good MIDI for older games, you need a GM Compatible card, a GUS or a Roland MT32. These work fine in a 486/pentium 1. If you want to run early 3D games I recommend getting a 3DFX card - a Voodoo 2, 3 or Banshee since quite a few early 3D games were Glide-only (3dfx Carmageddon, 3dfx Descent, GL-Qake and so on). If you just want to run old games and are not particullar about MIDI, 3D or quality / speed, turn the 486 into a DOS machine and the Athlon XP or socket 478 machine in a windows 98 rig.
The 486 is useful for older DOS speed-sensitive games IF it has a working TURBO function. If you get a Roland MT32 or a General Midi compatible ISA card, lots of old games have awsome MIDI music. I personally didn't know how good MIDI could sound until a year ago when I first tested my Yamaha SW20, and I was blown away. I didn't even know MIDI could sound so good since I couldn't afford a general midi card back in the day.
The pentium 1 would be a good starting point for a Glide rig. Upgrade the CPU as much as it will go (Pentium 200 or 233MMX) and throw a Voodoo 2 in there. If you like old Glide games, this build is a start.
As for the Athlon / P4 - see witch one is the fastest, get a fast AGP video card with win98 drivers and build an extreme win98 rig.
The rest have no use as retro gaming rigs in my view.