# kdesu gvim x
kdesu: cannot connect to X server
, you cannot use kdesu (are you running KDE or Gnome?) like this.
You need to be running as a regular user, then for KDE:
tred@vaio:~$ kdesu kate x
or for gnome:
tred@vaio:~$ gtksu kate x
If you are in a "real" terminal like <CTRL><ALT><F1-6> (and not a terminal window in your GUI) then neither method will work for you.