Ubuntu is excellent. I, for one, recommend you try the Hardy Heron (8.04) release. It is not quite out yet, but ready enough (release in 4 days) that I would be delighted for it to be someone's first step into desktop Linux. It also comes packaged on a Live CD, so you can see whether it works (or you like it) ahead of time.

As for DOS experience, there is one thing I feel I should mention. DOS has given a bad connotation to command line interface, and has given people an assumption that it is bad; they simply do not expect the great features available in modern shells.

For example, the Dash shell used for terminals in Ubuntu has contextual tab completion for practically everything. If you enter apt-get, it will tab complete for one of the operations that program does, such as install. Then it will tab complete actual packages, depending on the operation selected. That shell is simply incredible, and most distros use either it or Bash.

Oh, and the GUI is nice, too