|No you won't automatically get a prompt to choose operating system.|
Once you attach the 98 drive you may be able to go into bios and change the boot order to select the 98 disk as first boot device. You would have to alter the bios each time you wanted to switch operating systems.
You should be aware that 98 might have some trouble booting up in the different hardware environment although not as much trouble as XP would have if you moved that drive to a new system.
Is your XP drive formatted as FAT32 or NTFS?
If it is NTFS then when you boot 98, the XP drive will be unreadable.
My personal recommendation to you would be to simply attach the 98 drive. Boot up XP. If there is anything on the 98 drive you want such as documents, files, music,etc, then copy it to your system. then format the 98 disk and use it for storage space. Forget about dualbooting.
Goin' Fishin' (Some day)