If the game uses Steam, then you can use it to back up the game to DVD after it's installed - it will split it up onto separate DVDs, (or user specified size), and provide an executable on them to reinstall it. You could just back it up to the ISOs, then store those on a separate HDD mounting them to restore when necessary.
This is what I've done with CoD: Black Ops - downloaded from D2D, installed and then backed up using Steam.