I am running 9.04, too, 64-bit. I tried your suggestions with no luck. For some reason, the resolution and refresh do not change even after replacing the amdpcsdb file and applying the changes. Xrandr is still an available command option and it shows the max resolution being the 2048x768 at 60 Hz.
Is there a way to tell X whether through Xrandr or somewhere in the amd/ati config files that these values are higher? The xorg.conf file doen't have an option for "Virtual", and when I insert, save, and reboot the wallpaper has changed, window switching/minimizing/maximizing/closing is slow by 1-2 seconds, and the settings in both Catalyst and Gnome Screen Resolution remain the same. I can't seem to get the code to work using Xrandr: I need to add the resolution and refresh rate to be available in Catalyst and the syntax is a little confusing. Only 2048x768 shown when Catalyst open, but numerous under the Gnome Screen Resolution option under System=>Preferences, none higher than 2048x768.