I would install 98 first. Make sure your drive partitions are the right File system type :
Fat32 for windows Partition
NTFS for Windows xp Partiton
Fat32 for Data partiton
Windows 98 CANNOT read NTFS partitions at all, so if you want to be able to access all partitons from windows 98, you will need to use Fat32. You will lose a good portion of advancements in windows XP by using Fat32 unfortunatly, and it will create a slight aura of overhead.
Tough to call whether to install Linux or Windows first in that scenario. Just remember if you have both on the same system, be careful with Linux. There are few roadblocks to protect you from a simple mistake. You never forget the day you type rm -rf / It doesn't even ask you yes or no, just deletes everything. You will never make the mistake again, but you always remember it.
Which ever way you install the OS's, you need to change the boot loader and file to recognize the other OS. I have found that GRUB in Linux works better than Windows bootloader.