JWL
I just want to clearify a couple of things;
Nero backitup will automatically span more than 1 disk ( It will tell you how big the file will be ),for a large back up file it will notify you when to change disks (Good idea to mark the disks as you go)
Although Nodsu has a point about retrieving a backup when your system/Nero is down .What you asked for is a 'data' backup ,which you can retrieve because your CD 's are saved in UDF/Multisession format and can be read and copied in Explorer.And you are presupposing you have an operating system working .
Nero or most Any authoring software automatically reads a blank disk and registers how much data can fit,,With Backitup you don't have to be concerned about the size of files or if they will fit .Just make sure you have enough CD's ready.
For a backup for your O/S I would recommend Drive Image so you can store a complete image of your drives ( preferrably on a seperate hdd or external drive)