Read
this link to figure out what files you need to save from your XP drive so you don't have to fight Microsoft on reactivation.
Then wipe your XP drive (20GB) once you've gotten everything off that drive you want.
Then build your new SLES on the first drive (20GB). Do not connect the 6GB drive yet. After your new installation is complete, then you can install and mount the 6GB drive. Save the files on your 20GB drive from the 6GB drive.
Then install XP on the 6GB drive (20GB drive removed).
Now you can put the 20GB drive back into the computer as the slave drive. Modify the boot.ini file for XP so you can boot Linux.