I agree that a Pentium 4 is overkill for most Win98 games, however this is not always the case. Heck, I have two WinXP/Win98 dual boot machines with Core 2 Duo / Pentium Dual Core CPUs.
The truth of the matter is that, surprisingly, there are some Win98 games which are CPU bottlenecked even on an Athlon XP 2500+ (NFS High Stakes is one such example, which is probably poorly optimized - very hard to achieve a steady 60+ FPS)
On the other hand, I think that a Pentium 4 can actually struggle with newer Windows XP games (and even some older ones, like Far Cry). So, to me, P4 makes sense either as a Win98 only PC or as a dual boot WinXP/Win98 PC (which would allow you to play basically all Win98 games + early Win XP games). But for an XP-only machine, I think there are much better options than a Pentium 4.
And as for video cards, an FX 5900 / 5950 (or even better, a Radeon 9800 PRO/XT) do make a lot of sense if you simply want to max out everything including Anisotropic Filtering, Antialiasing and also run late Win98 games @ 1600 x 1200 or similar resolutions.
Not saying it's something that everyone wants to do, many times I myself simply prefer to play games on a SS7 PC, at < 30 FPS, just for nostalgia purposes 😀 .
1 x PLCC-68 / 2 x PGA132 / 5 x Skt 3 / 9 x Skt 7 / 12 x SS7 / 1 x Skt 8 / 14 x Slot 1 / 5 x Slot A
5 x Skt 370 / 8 x Skt A / 2 x Skt 478 / 2 x Skt 754 / 3 x Skt 939 / 7 x LGA775 / 1 x LGA1155
Current PC: Ryzen 7 5800X3D
Backup PC: Core i7 7700k