A few thoughts from someone who has an old Win98 rig still functional and being used occasionally.
In practice, I found that dual-booting DOS and Win98 is in most cases unnecessary, since Win98 runs on top of real mode DOS. I have yet to encounter an application that in principle runs under DOS 6.22 but not under "DOS 7" (the code name of Win98 DOS). Such applications do exist, I've heard, but they are far from mainstream.
I am not sure about using T42 as a Win98 rig. It is probably somewhat overkill, and compatibility will not be at its best with many drivers possibly missing.
The question is also - what Windows games can you run on Win98 but not on WinXP? I haven't encountered many of these, and when I did, I was not sure whether it's really a problem with OS compatibility or with video card compatibility (quite a few old games were geared towards 3dfx/Glide, and their DirectX modes were poorly supported).
Of course, if the goal is DOS games, it's a different story, because Win98 has a native DOS mode, while XP or anything NT-based for that matter does not. But then we are hitting the "hardware too new" problem again, with questionable sound/video card compatibility.
My experience with getting PCI sound solutions to work under DOS was unsuccessful. Apparently, like jdrou says, they are supposed to have a Sound Blaster emulation mode, but it does not always work. In my case, I was using a PCI SBLive Value card, and Creative's SB emulation utilities, but still couldn't get the DOS games to detect the card. I read somewhere that it was because my motherboard did not support non-maskable interrupts or something. Not sure what the status would be with the sound solution on a T42, and where one would even start looking to enable their SB emulation mode.
Current: T410 2537-R46, T60 1952-F76, T60 2007-QPG, T42 2373-F7G, X61 7673-V2V
Collectibles: X32 w/ IPS Screen, A31p w/ Ultrabay Numpad
Past: Z61t 9440-A23, T60 2623-D3U, X32 2884-M5U