A Socket 😄 478 motherboard by SIS - any will do but SIS chipsets allow for PCI soundcards to work better in DOS.

Literally 😄 any socket 478 Pentium 4 CPU is "good" enough for Windows 98 and fast enough (or faster) than the CPUs 😄 people were using at the time (up to 1.4GHz Pentium 3) (but make sure it is compatible with your board 😄 - buy the board first!). I would aim for 2GHz and up, and these are easier to find than the 😄 older slower ones.

Literally any DDR1 RAM. The speed doesn't matter because any DDR1 RAM is faster than what was "good" 😄 for Windows 98 (133MHz SDR RAM). Only use <512MB because that's what Windows 98 supports natively. Windows 98 games generally 😄 use a max of 256MB and I would be quite happy if my Windows 98 PC had 128 to be 😄 honest. You can patch Windows 98 to use more ram but the patch brings about issues of its own and 😄 considering that Windows 98 and its games don't need the RAM, I wouldn't bother with it at all.

Sound cards are 😄 tricky - yes you can spend lots of Yamaha PCI sound cards but a loved/hated Soundblaster Live (or Audigy1) will 😄 give you basic sound in DOS and good sound in Windows games. They're dirt cheap and readily available, but you 😄 might be waiting for a while for a Yamaha to crop up.