Very good questions and great idea asking them before you hose everything up
1. Make a Linux Boot floppy.
2. Make a Partition Magic Rescue Floppy set.
3. Make a WINDOWS 98
emergency boot disk. (can download am image from the net I'm sure if you don't run 98 anywhere.
4. Do what you need to do in partition magic.
5. If nothing will boot, boot up from the win98 floopy and type format /mbr. This will allow you to load windows and come here for info on how to fix GRUB.
Chances are good everything will be just fine. I've changed several systems around the way you want to and haven't had any serious problems.