You need fast DOS system for hi-res unaccelerated 3D texture-mapped DOS games. Popular titles like Duke Nukem 3D and Blood are remade in OpenGL, so you don't need fast DOS hardware for those. I'm talking about lesser known games like F-22 Lightning II, Dawn Patrol, Top Gun: Fire At Will with -superhi option, Azrael's Tear, and the first System Shock. I've tried Azrael's Tear and Dawn Patrol using DOSBOX, running on 2.4 GHz Intel Core i5 CPU, and frame rate is not smooth. The choppiness doesn't hamper gameplay, but it's noticeable. F-22 Lightning II is another thing though; its choppiness is enough to hamper gameplay that it feels like playing on Pentium 100 again. As such, the needs may vary. A Pentium III 600 should be more than enough to run Azrael's Tear silky smooth, but I guess you need something like Pentium 4 to run unaccelerated Quake in 1024x768.
You don't need fast DOS system for hi-res, unaccelerated, non-textured (or barely textured) 3D games, nor do you need it for low-res (320x200) textured 3D games. For such games, DOSBOX will suffice. Star Rangers runs fine on DOSBOX.
Then, Windows 98. The person I knew to run the fastest Windows 98 system is Malik; he runs it on a Core 2 Duo CPU with GeForce 6800 GT PCIe, if I recall correctly. One may wonder why would you need such fast system for Windows 98. After all, most Windows 98 3D games are GPU accelerated, either with GLide, Direct3D, or OpenGL, so the need for fast CPU actually becomes less important. However, there are actually games that benefit from faster GPU, but are problematic on XP. Jane's USAF is "new enough" to benefit from faster GPU, especially when you want AA, but the game needs patch to run on XP. Jane's F/A-18 is also problematic in Windows XP, yet the game actually needs fast GPU and CPU for silky smooth frame rate.
Also, there's Aureal 3D consideration. I've never tried Aureal 3D sound cards on XP, but I read it's problematic. So if you want to experience Half Life 1 with a real Aureal 3D sound card, then you'd need to build a pretty fast Windows 98 system.
Never thought this thread would be that long, but now, for something different.....
Kreshna Aryaguna Nurzaman.