As Isotope has already said, it's very easy to do - when you install either 2000 or XP it gives you the option of overwriting your existing OS or a new install - it even creates an option menu at start up.
Installing Linux does the same, it would be quite possible (and easy) to have three options at start up, 98, 2000, and Linux.
One thing to be aware of is the different disk formats, 2000/XP will be able to read the 98 disk, but 98 won't be able to read the 2000/XP disk. With 2000 I think you may be able to set it to use the Win98 format, but I think XP will only use NTFS - but I'm no expert on 2000 or XP, Win98 rules :twisted:
I do have a dual boot machine with 98 and XP (main boot in 98), basically for checking WinPicProg compatibility under XP (none of the NT based Windows appear to work quite right! - a perfectly legal Delphi program wouldn't work, and required modifying - it was a really obscure problem!).