If you truly want to learn UNIX I would suggest one of the BSD's or Slackware Linux and preferably good book on UNIX System Administration. But get ready for a long period of sleepless nights
If you'd just want to get started quickly and *really* need GUI in your daily use, any of the popular Linux distros (Mandrake, Red Hat) will do this job great.
But really, the more newbie-friendly distros can be a nice gateway into "real" UNIX..
I started of with Red Hat 5 and eventually moved over to Slackware, and later setting my foot in the BSD's as well. In other words, thank you Red Hat for introducing me to the Linux/UNIX world
Good Luck