The latest official ISO is available in Step 1 of Clean Reinstall - Factory OEM Windows 7,
along with tool to burn to DVD or write to flash stick for reinstall or repairs. The tutorial also assures you'll get a perfect install even for retail.
A reliable way of backing up installer disks is storing their ISO in your backup to burn to disk or write to flash stick when needed. However if you have no other PC handy to burn the disk if needed for repair or reinstall I'd have the latest installer disk or Repair CD on hand.
To extract your DVD's ISO (I'd only bother if it has SP1) use ImgBurn
to extract the ISO, then burn that ISO to DVD using ImgBurn at 4x speed. This is how you copy the DVD, too. So you might as well back up the ISO since it needs extracting first and you'll have it.