You can redownload as many times as you like by using Impulse. So, if bandwidth isn't a problem, you don't even need to burn a backup.
But if you want to, just right click on Sins in Impulse and select archive (the default path for the archive storage can be changed in blue orb > preferences). Then burn that archive on your DVD, and you'll be able to install from it once you install Impulse on your freshly formatted HD 
Hope this helps!
