To install GUI

Code:
sudo aptitude install xorg
then either one of gdm, xdm, or kdm.

Code:
sudo aptitude install gdm/xdm/kdm
then either xubuntu-desktop, ubuntu-desktop, kubuntu-desktop. Or xfce/gnome/kde/openbox/etc I use xfce so I did.

Code:
sudo aptitude install xfce4

The difference between xfce and xubuntu is that xubuntu comes with more apps. I think the same applies to gnome/ubuntu kde/kubuntu.

If you decide to go the Xubuntu route, but choose to install the xfce4 package make sure to installing thunar alongside it, otherwise you'll have no file browser.Edit: Although xfce4 might install Thunar automatically, it's probably safer to install the xubuntu-desktop metapackage.
The command to reconfigure the x-server is

Code:
sudo dpkg-reconfigure xserver-xorg