Shell confusion

[Dave Bolt IT Solutions]

When I start a terminal as dave I get a prompt [dave at Test-Host ~]$
When I do su amandabackup I get a prompt bash-3.2$
>From the first prompt I can launch gedit, from the second I get an error, No 
protocol specified, gedit:4724, Gtk-WARNING ** cannot open display.
Since both of these users are apparently configured for bash, can anyone 
explain what is happening?
Or where I have to look to see the difference in configuration?
Thanks
Dave