Which version of XP are you running? Home or Pro? If it's Pro, you can do it, but your going to have to reinstall XP, since installing 98 is going to mess things up with your drive. If memory serves me right, it wipes you MBR, so basically, your computer doesn't see whatever other OS you had on. Also, I believe that you must have 9x on your bootable partition.
So, if your using XP Pro, you'll need to install 98 on your bootable partition, then install XP Pro on a secondary partition. Once XP Pro is installed, you'll get the NT boot manager giving you the option to boot in 98 or XP.
If you want to run XP Home and 98, then you might run into some trouble. You will need some third party boot manager to do this, and if both 98 and XP Home have to be in the bootable partition, then your going to have to install them in the same partition which could be problematic. If that's what you're going to do, I'd recommend you install them in different Windows directories, for example, C:\Windows for 98, and C:\WinXP for XP.