If you want to do TRUE backups, use DVD decrypter to create an ISO of your DVD movie. This will be an EXACT duplicate of the original DVD. If you want to watch it on your PC, just mount the ISO using Deamon Tools (or whatever virtual drive program you like) and play it using PowerDVD (or whatever DVD player you'd like). The down side to this is that you'll need ALOT of hard drive space. Each DVD can be from 4-8+GB.
If you want to back it up and shrink the file size of the movie, use DivX or Xvid. This will give you very nice quality in a much smaller file (usually a good quality movie will be around 1000-1800MB). The down side to this is that you'll only be able to watch it on your PC unless you have a video card that will output to your TV.
Finally, you can use SVCD. Not as good as your original DVD, but it will play on alot of stand alone players.