It depends on what you want to do with it. The ultimate solution for everything doesn't exist.
A K6-III+ is good for downclocking and makes an interesting combo machine for early Windows + DOS. But it lacks power for the latest Win98 titles.
I wouldn't recommend using a Voodoo3. It has issues with some 2D games and early Glide games. GeForce2 + Voodoo2 SLI gives you more flexibility, but will also cost more.
Pentium III is nice, because the boards still have ISA slots for old sound cards. But it lacks the downclock capabilities of the K6-III+. So for DOS it's kinda meh.
If DOS and ISA aren't an issue, you're better of with an Athlon XP Barton on KT333 chipsets. Those are the last which still support AGP 3.3V. Add a Voodoo5 5500 and you have the ultimate late Win98 system.
Another viable option are the mentioned AGP boards from ASrock which can run a Core2Duo/Quad or even a Phenom II (ALive Dual, AM2NF3-VSTA).
Here's an interesting video about them:
But you won't be using an AGP Voodoo on these boards. GeForce FX (5950 Ultra, if you can afford it) is the GPU of choice for these boards.
But you can add Voodoo2 SLI for Glide. Or even a PCI Voodoo5. 😁
For sound cards the choice is pretty clear:
Soundblaster Audigy 2 ZS (Platinum Pro) for EAX and Vortex 2 for A3D. You can use both in the same machine.
If you're looking for compatibility in graphic cards, ATI is NEVER the answer!
They're a pain in the ass. Speaking from personal experience back in the day.