As mentioned. MacTheRipper is good for copying the DVD from the disc to your computer. Most DVDs today are copy-protected, which is why you need special software for this.
Now, you can leave them on the computer or on an external harddisk OR burn them back onto a blanc DVD. The latter needs compression. Since most movie-DVDs are DL (dual-layer) and around twice the size of you regular blanc burnable DVD, a movie simply wont fit onto the blanc disc. You need to compress it in size. This can be done with DVD2One or Handbrake or Toast. If you dont already have any dvd-cd-burning software, Toast is very good, and will cover most needs.
Get MacTheRipper and Toast, and you're good to go.