Just remember that you're talking 1998, AGP didn't come out until late 1997 and was still not mainstream in 1998 as most gamers and casual gamers were doing just fine with dual 12MB Voodoo 2 cards in SLI mode. At that point in time AGP was still a niche thing and mainly only hardcore gamers were all about it, it wasn't until 1999 that AGP became more mainstream and the casual gamer began to want it.
When it comes to AGP vs PCI then AGP 1x was identical to PCI 2.1.
You stated that you'll be installing 2 Voodoo 2's in SLI anyway so any PCI 2D card will work. Get yourself a nice S3 ViRGE or S3 Trio/Trio64 chipset PCI 2D card and you'll be golden. Diamond made some of the best ones for those chipsets as did Tseng. Or simply get a Voodoo 3 3000 PCI that way you have Direct3D, OpenGL, Glide and 2D support all with in one single card and a bit better performance than Voodoo 2 SLI. Voodoo 3 3000 came out in early 1999.
BTW the fastest CPUs that the ASUS P2B98-XV supports is Slot-1 Celeron 533 @ 533 Mhz, Pentium II @ 450 Mhz and Pentium III @ 600 MHz -- if you want to stick with 1998 era specs then get yourself a Pentium II 450 Mhz because Celerons from that era were really bad performers. If you're willing to push to 1999 specs you could max out the board with Pentium III 600 Mhz as that is what most people would have eventually done anyway.
Having a period correct system from that era is difficult as the technology was rapidly advancing, so IMHO the best way to go would be a 1998/1999 hybrid; best of both worlds both technology wise and performance wise. 😎
All the world will be your enemy, Prince with a Thousand Enemies, and when they catch you, they will kill you... but first they must catch you. 😁
