iDVD creates a disk image from digital video files (like from iMovie or Quicktime). Doesn't help with DVD ripping (unless you use Macthripper then use Handbrake to make the video file then use iDVD to create an image).
probably best to just use Mactheripper to get the Video_TS file and keep that on the hard drive. (and yes, in the USA even if you own the DVD, the legality of this process is at best debatable).
EDITED on account of me being stupid 🙂