Dual monitor with ATI

Question

I'm running 12.04 dual booted on a Dell Inspiron i17R with an ATI 5400. I'm trying to get an external monitor (Dell UltraSharp U2412M) to act as a second display. When I connect it to my computer, it happily mirrors the display, but does so at 1440x900, not the 1920x1200 I know it's capable of. I want to disable mirroring, and I want to up the resolution (I suspect the latter will be easy to take care of once I do the former).

When I go Settings-->Displays and uncheck "Mirror displays", I get a dialog box with an error message:

The selected configuration for displays could not be applied

requested position/size for CRTC 148 is outside the allowed limit: position=(1600, 0), size=(1440, 900), maximum=(1920, 1920)

Clicking "Close" brings up another dialog box which says:

Failure to apply configuration: %s

GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._gnome_2drr_2derror_2dquark.Code3: requested position/size for CRTC 148 is outside the allowed limit: position=(1600, 0), size=(1440, 900), maximum=(1920, 1920)

I'm aware that this question has already been asked before. Following that solution, I try

sudo amdcccle 

but when I select Multi-display desktop with display(s) 2 and click 'OK', my terminal returns the following error messages:

(process:3491): GLib-GObject-CRITICAL **: /build/buildd/glib2.0-2.32.3/./gobject/gtype.c:2722: You forgot to call g_type_init()

(process:3491): GLib-CRITICAL **: g_once_init_leave: assertion `result != 0' failed

(process:3491): GLib-GObject-CRITICAL **: /build/buildd/glib2.0-2.32.3/./gobject/gtype.c:2722: You forgot to call g_type_init()

(process:3491): GLib-CRITICAL **: g_once_init_leave: assertion `result != 0' failed

(process:3491): GLib-GObject-CRITICAL **: g_object_new: assertion `G_TYPE_IS_OBJECT (object_type)' failed

and no change occurs. Any help would be greatly appreciated.