To dual boot between 2000 and 98, you should have at least two partitions, or two drives (physical).
From Windows 2000, you will be able to install 98 to another drive, (do not install 98 onto the existing drive C
You will then have a boot loader to select which OS will boot at startup. (generally allowing 30 seconds to elapse before the default OS will boot, in this case, Windows 2000).
If you don't see a boot loader, you'll need to modify/edit your boot.ini file
So you'll be requiring different hard disks (say C and D), or two different partitions (again, C and D) on the same physical hard disk.
I've never had to dual-boot between 98 and 2000, although, had I wanted to, I would install 98 beforehand, and allow Windows 2000 to arrange the boot sequence/boot.ini for me (personal preference, I would imagine that 2000 has an 'advanced' boot manager than 98 does when it comes to dual-booting OSes).
Once you've got 98 installed, boot up into 98, pop in your Windows 2000 CD, and you'll be prompted to upgrade, select Yes. Once you're in the wizard, you'll be able to select which partition will be used to install 2000 on, so you'll want to install a clean copy of 2000, (which should be something like: "Install a new copy of Windows 2000 (clean install)".
From there, you'll need to get into the Advanced Options where you'll find "I want to choose the installation partition during setup".
That should be it, but before you do anything, I should tell you that Windows 98 (if on a FAT32 filing system), then you wont be able to access your Windows 2000 (NTFS) file system, (unless, of course, they are both FAT32).
Once 2000 is installed, your boot.ini file will be configured to suit.
What you want to do isn't recommended (personally), as Windows 2000 will support games, can run in compatibility mode (for Windows 95, 98, 98SE, ME. etc).
Although, as I've said, I have never tried to install 98 and dual-boot with 2000, AFTER installing 2000 on it's own. (I have clean installed 2000 whilst on 98 however, to another partition).
Hope it goes well.
Good luck