Do you have X11 running? That could be a problem if you dont have that running. Also if you are in kde and want to run them why dont you just goto the run choice in the menu and put it in there?
Before you type the "su" command type "xhost +localhost" which will give all users on the local machine permission to run X apps on your X server. Or worse yet just "xhost +" which will allow anyone anywhere to display X apps on your X server. By default only the logged on user on the local machine has permission to display X apps on the local server. This will stay in effect until X is restarted or you take away permissions with "xhost -xxxxx".
Other distros set up the X authentication automatically when using the su command. I would have to dig through the scripts to see exactly how Red Hat does it, don't have time right now though. Probably using the "xauth" command in some incarnation.