Putting a WIn7 Backup Image on DVD's is laborious and wastes a lot of disks.
An external drive is a much better way to back it up.
You can use a CD for the Win7 System Repair disk, or if you have a Win7 DVD then the Repair Console is built into it already.
And if you have the 100mb System Reserved boot partition assigned during installation, the Repair Console is also added to the Advanced Boot tools accessed by tapping F8 at startup.