AGP 2x, even 1.0, should twice as fast as a typical PCI slot, but then it comes down to how much of that bandwidth the GPUs can actually use.
8400GS was available as a PCI card.
9400GT and 9500GT were also available as PCI.
These were likely a lot slower than the PCIe versions, and were certainly intended as the last gasp for AGP.
These are much newer than any of the 8x AGP Radeon 9000 cards, but putting an AGP 8x card into a 2x slot may have adverse effects.
Probably just want to look for a better board or simply try running DOS and Win98 on newer hardware.
Around that time I had DOS and Windows XP dual booting on an Athlon X2 with an 8800GTS PCIe card. Didn't have any issues running very old games on that hardware. Most Win98 class games will run just fine in compatibility mode under XP as well, perhaps a little tweaking.
Quite a few old games actually run alright in Windows 10 and 11, but there are some that flat out refuse. Though I understand many can be thrown into a glide wrapper that does a good job of getting old OpenGL/OpenCL capable games running.