Win98 the best DOS gaming setup? Would want to disagree with that. You're looking at memory management issues (try Ultima 7 on Win98...), the ideal sound cards are completely different etc etc.
Yes, it's possible to do an early to mid Windows 98SE build that also can handle DOS, but you're looking at compromises on both sides there. I'd argue the exact opposite: any "Windows XP" game too new to run on an XP/98 box can just run on a current Windows 10 machine, so no need to have a souped-up XP box unable to handle Windows 98. Tbh, the only added value I see in WinXP builds is that you can still run 16b Win3.x games on them...
So, back to OP's original question. For DOS games, the question is which era - there's a massive difference between Alley Cat and Quake. It's - sort of - possible to cover all of it, but if you want to go down that road you need to know it from the start and subordinate other choices to it. It's not just CPU speed, also sound and display options.
My main DOS games rig is aimed at 1989-1994 games (simply not that interested in older stuff, and for late DOS I have a different build - also if I change my mind about <1989 I have an Olivetti M240 XT with EGA to cover that). It's currently configured with a UMC U5S-33 CPU (a fast 486SX33 clone), UMC 85c418 VLB VGA (excellent raw DOS performance, although you don't want it in any GUI), an Aztech MMSN810 Sound Galaxy Basic 16 (1st gen Aztech, with Covox and DSS as well as AdLib, SBPro2 and WSS), Music Quest intelligent MIDI card and I intend to add my PAS16 if I can ever find the damned thing... The goal is ideal performance in Ultima 7, and for that I've boosted RAM to 64MB, so that I can have a 32MB Ramdrive to load the game onto before running. This is plain silly, for any normal purposes 8MB RAM would be more than enough for a system like this. But hell, I have the RAM so why not? Just remember to disable HIMEM.SYS ram check or boot takes a loooong time. With turbo off, the system runs at early 286-ish speeds, which is more than slow enough for the bottom end of my target. Storage is handled by CF cards, using a 2GB card for most stuff.
I used to have a more 1991-1996 build, based on a Pentium 200MMX. For sound I had a GUS and an SB16 with real OPL3 (iirc CT2910). However after I built a K6-2 together with my son to play OMF2097 (quality father-son bonding 😜 ) this P200MMX build was surplus to requirements, so I moved the GUS and an AWE64 Gold to the K6-2. No real OPL3, but with 1995 and later focus that's not an issue.
Now, my Win98SE system is a different beast. Pentium 3-1400S (Tualatin) on an ECS P6S5AT (SiS 635T DDR motherboard) with 512MB RAM, a GeForce3Ti200 and a Turtle Beach Montego II (Aureal Vortex 2) sound card. Storage is via SATA adapter and an SSD. No ISA and not missing it. Plus if I want to run Windows XP (which I don't tbh) this system can handle it easily. Again, any XP game that a P3-1400S can't handle will run on my modern i7, so not missing anything here (and yes, I still play CnC Generals ZH regularly in that way 😉 ). If you disagree and want more muscle, so long as Win98SE can still run, it will work. I'd say head for an i865-chipset system then, perhaps an Asrock version that can handle Conroe CPUs. But for me, that's more a slow modern system than something retro.
Bottom line: notice how often I say "I" or "for me". The only person to decide what you aim for is you. Once you have that clear, other people can help you with how to technically implement it.