No problem dual booting w/ xp.
If your worried about 98, try making an image of your 98 partition. Also make a copy your boot.ini and ntldr.exe from your primary drive. Better yet, make an image of your primary partition.
If you have any problems, you could alwyas restore those images and you'll be back where you were before installing 2k3.
Power quest makes a good imaging program. It will create compressed images that you can save to cd. (My primary partition - xp w/ programs - fits on 5 cdrs.) Acronis also makes a good imaging program.