Add me in as another one in favor of the Intel i865 integrated graphics (Extreme Graphics 2).
It should be very decent (if not great) for most of late 90's games. Half-Life and Counter-Strike 1.5/1.6 (non-Steam) for example, run butter-smooth at 60/72 FPS almost all the time.
I would dare say i865 is quite OK even for some early 2000's games (Mafia, GTA VC, Collin McRae Rally 2/3/4, all NFS series up to Hot Pursuit 2 [haven't tried NFS Underground or Underground 2 yet] - these are just to name a few that I tried and can confirm they run well).
And since that chipset is usually bundled with a socket 478 platform, it should make for a nice Windows 98 build, especially with a faster P4. I haven't tried it yet myself, but do have a few systems I eventually plan on trying it with. Currently, all of these have some form of Windows XP on them (including the one I am posting from right now, which I still use for occasional browsing.)
I haven't tried anything on the ATI Xpress 300 or Xpress 1000-series IGPs, but I do have a few motherboards with ATI RS482 chipset (Radeon Xpress 200 IGP), and it seems to be about on par or slightly better than i865. The main difference between the two is the DirectX feature level support - i865 is only DX7 -compliant, whereas x200 is DX9(a/b) -compliant... which shouldn't matter too much with late 90's games, as most were still DX7-based. Also with X200, I seem to get overall more stable (and higher) FPS in most games. As for OpenGL performance, I don't remember if I tested it or not anymore (been a while.)
On the nVidia side of things, I recommend to stay away from any hardware with nForce 4 and later chipsets, as most of these are not reliable chipsets. The GeForce 6100/6150 series of chipsets are probably the worst in that regard. If you have one, make sure to put a fan (small or big, it doesn't matter) on top of the heatsink to keep it cool. This should help prolong its life quite a bit. Otherwise, from a feature standpoint, I don't mind them at all. Not sure how the driver situation is with the GF 6100/6150 chipsets and Windows 98. Anyone tried it and care to share some details?