Anything will do for pure dos, and it doesn't have to be fast.
Socket 7 / super 7 advantages:
+ CPU speed can be set up by jumpers. You can run a 400MHz K6-2 at 100Mhz if you wanted to. That + disabling L2 cache emulates a 486 pretty well.
+ Some boards / chipsets have funky undocumented jumper settings for setting the FSB. The VIA VP3 for example can be put in a "troubleshooting mode" where the CPU, RAM and Cache all run at ISA bus speed (8MHz). That makes it great for emulating old XT / AT machines witch are a pain in the ass to come by these days. I've managed to get a K6 running at 8, 20 and 40Mhz on a VP3 based board - that makes for a pretty nifty all in one retro machine.
+ They're fast enough to run some win95/98 games. A 500MHz K6-3 has no issues playing quake 2, Half-Life and other games from that period when paired with a decent 3d accelerator. A Voodoo 3 for example is great both as a 3d accelerator, but in DOS as well. No graphical glitches or tearing.
+ (subjective) almost all socket 7 / super 7 boards are AT format - having a DOS PC in an AT case with a turbo button and MHz display just feels right for me. AT format motherboards can also be used in ATX cases - this is a big plus since AT cases are hard to come by in some parts of the world, or are pretty expensive.
+ most Super Socket 7 motherboards have an AT and an ATX power connector. This allows you to use an ATX case + PSU, or an AT case (for that vintage look) + an ATX power supply
Disadvantages:
- Slower then slot 1 / socket 370 chips - especially FPU performance. BUT - very few dos games even make use of a FPU, and some games will not run properly on a very fast machine like a 1000Mhz P3.
- K6-III chips are pretty hard to come by
- some Super Socket 7 motherboards are pain in the ass to get running correctly. Cheaper boards are poorly built and are unstable. Driver support is hit and miss under win98.
Socket 370/Slot 1 Advantages:
+ Cheap
+ Fast
+ Easy to come by
+ Fits in a ATX Case
+ easy to get running
+ very stable platform
Disadvantages:
- Slot 1 / Socket 370 builds are in an "awkward valley" when it comes to usefulness - at least from my personal experience. For all 3d accelerated windows 95/98 games, a pentium 4 / Athlon XP will work just as well as a pentium 3 - even better for some more demanding games if you want to run them at 1600x1200 and force anti-aliasing and anisotopic filtering with around 60 FPS with no frame skipping (yes, I know I'm being picky and most people would be happy just having the games to run). I find that a P3 machine is just too slow for some games - some examples being Homeworld, Populous 3, Uprising 2 and Dungeon Keeper 2 - they will stutter a lot when there's a lot of action on screen even when using a 1400Mhz Tualatin @ the afformentioned resolution / detail levels.
- Some dos games are impossible to run on fast machines, and pentium II/III chips are multiplier locked - so running Ultima or Wing Commander will be difficult. Even commander keen 4 displays some odd behavior on machines faster then 500Mhz (stuttering).
I have a couple of pentium II/III machines - one is an ABIT BE6 based build. I initially ran a 400MHz P2 on it, but I later switched to a 333Mhz chip that can do 500Mhz stable. The other two are an Abit ST6 based 1400MHz tualatin with a chaintech 4600ti and a 1000Mhz P3 running on an Asus CUV4X-M with a Geforce 3 Ti200. The latter is a mATX build. The only game I found really benefits from running on one of these machines is Duke3D in VESA mode at 1024x768. I haven't used any of these machines in months due to the reason I mentioned above, plus they're rather mundane... slot 1 / socket 370 machines make me think of office computers , not home/gaming PCs.
My "daily driver" dos PC is a 400MHz K6-2 running at 66x6 on a Lucky Star VIA VP3 board. The video card is a Voodoo Banshee PCI, and the sound card I chose is a Guillemot Maxi Home Theater 64. I found some undocumented jumper settings (by trial and error) and I wired up some dip-switches to the back of the case witch let me run it at 8, 20, 133 and 400MHz. I use SetMul to disable L1 and L2 cache by running batch files at startup via a boot menu (fun fact - the PC will freeze when loading himem.sys unless I disable L2 cache - dunno why). This PC runs all of my favorite DOS games as well as some 2D windows games (C&C, Red alert, Starcraft, Age of Empires, etc). As for dos games, it runs even picky games perfectly provided I set it up for the game beforehand. Dyna blaster for example refuses to run correctly (completely ignores keyboard input) if the system is clocked higher the 20Mhz. Other games like Lode Runner, Paganitzu and Digger expect an 8MHz XT and will run way too fast at anything over 8MHz.
As for win98 era 3d games, I use the aforementioned GX400 Dell + a radeon 9700 and a Yamaha 724 with a 20" Samsung Syncmaster 214T.