I’d go with 40 on c: (system) and then make three more of equal size. Two for files, music, and such and the last one to hold those images that you are going to make as backups from now on.
Seriously, when you get done, get Ghost or something else that does the same thing and make a restore image. You can create one that spans several cd’s if needed and keep it as a master. They have a shelf life of at least 2 years if stored in a cool dry place. For working backups, just write them to the last partition. Compress them as much as you can and keep at least two. That gives you two points to restore from just incase you end up backing up something that you would like to be rid of.