The best way to do it is to install each OS in it's own primary partition.
You will need a good third party partitioning program like Partition Magic to do so, as FDISK will only allow you to create one primary partition.
Since you already have Win2000 installed, all you need to do is create another primary partition to install Win98 on. After you have that done, you will need to set that partition as the active partition. Install windows 98 as usuall.
Now, to switch between the 2 primary partitions you will need to install a boot manager program.
I use this one and it's free.
http://www.osloader.com/