I can't give specific help because I too use an nVidia Geforce 5200FX!
I agree with DethKnight, this usually happens when you are in the "wrong user" when you try to execute something from the console and the user you are in can't open another X display because there is already one running via the first user. Use "exit" to logout back to the first user and try again.
Another idea (and this MAY be hokey because like I say I use nVidia) would be to use sax2 to configure the display. *IF* it's done the same way as with nVidia you need to know the module that is used for radeon - this will probably be specified in the "Device" section of the file /etc/X11/XF86Config as
Driver "radeon"
or something like that. You can also use lsmod to check which modules are presently loaded to see if there is one called e.g. "radeon" that matches the XF86Config entry. *IF* it is done the same way as for nVidia you could then get sax2 to configure the display by...
doing "init 3" in a console to get to text login, login as root, then do
sax2 -m 0=radeon (that's a zero btw, radeon = whatever module was called)
