Windows 2000 and XP both multiboot fine, and in Windows systems automatically set up the autoboot for you.
If you want to use Linux etc. then you will need an extra bit of software (like BootIT) to set up the multiboot (although it is possible to use grub toi boot windows too).
If you want to keep a Windows 98 install make sure you install Windows 2000 afterwards (not first) as Windows 2k/XP are aware of Windows 98 and ME but not vice versa.
The best thing about upgrading to Windows 2000 or XP is stability issues. If you are going to pay for an upgarde why not go straight to Windows XP, it is basically Windows 2000 with a few extra bits, but MS will be supporting it for longer and better than 2000 (eg. Internet Explorer 7 is porbably going to require XP, which implies later security patches will be aimed more at XP).
Do you have children, work in education, know someone with children, ever met a child? Then you may find you qualify for Windows XP educational pricing if you aren't using the computers in a commercial environment.