Tramp su from dired

[notbob <notbob@nothome.com>]

As a novice emacs fan, I'm learning by using emacs primarily as a file mgr
and file editor.  I often need to edit a root file as a user and have
usually just opened up a new session of emacs as root, but would like to
utilize the tramp mode.  I've been able to edit a root file from via tramp
when not in dired mode, but when using e (edit) or C-x C-f in dired and then
specifying a file -- /su::/dir/foo -- I end up opening a new foo file
instead of the existing foo file I specified.  

Is it possible to do su from dired or should I just open up a non-dired
window?  If so, how?  (told you I was a noob)  ;)

nb