I'm starting to theroize that maybe motherboards and mobo drivers may have some impact on all this, or perhaps the win9x versions of the same drivers aren't exactly the same as their winx counterparts...
Oh, there's a very good chance they are different -- after all, Win98 prefers the .VXD architecture while 2000/XP prefer the WDM architecture.
Now, you'd expect that you'd be able to get the same types of features regardless of the architecture, right? Unfortunately, the WDM architecture is nerfed to heck and back under Win98; just look at what happens when you use a WDM sound driver in 98 -- virtually ZERO hardware acceleration for EAX, Aural3D, DirectSound, etc. is given in exchange for hardware DirectMusic support, and CD-Digital Audio Extraction, which are two features you can't get with VXD in 98.
....anyway, I suspect its a similar thing going on here with hardware other than soundcards (like motherboards, or video, or perhaps even network cards and modems) -- the 2000/XP drivers are written with a WDM approach, and won't do jack in 98, so they have to re-write them for the VXD specification, which will still have some limitations, so the drivers probably have a few (small....ish) things that they can't quite do in 98 that they otherwise could in 2000/XP.