If the new motherboard fits the old case physically, you are only just starting.
The PSU is probably going to have to work too hard for its own good and should be replaced for the higher power needed.
The RAM will almost certainly have to change from SDRAM to DDRAM.
If you want to use 98 at its first startup to upgrade to XP you may find that W98Se doesn't "understand" the new faster CPU and just fall over!
A pity, but I think it is not an economical jump and you might think of replacing the box completely and passing the old one, to someone with less demanding criteria.