when you open up normal emacs with DISPLAY variable set, it tries to create it's own window (and I repeat emacs, not Xmeacs). So you just have to figure out how to allow root from su session to conncet to xserver. look at xhost as advised.
try xhost + -- Do not meddle in the affairs of wizards For they are subtle and quick to anger Lord of the Rings - J.R.R Tolkien