Forcing monitor resolution in VirtualBox VM on Linux Guest-Collection of common programming errors

I am working in a VirtualBox 4 VM running Slackware-current. I have added an external monitor, and am attempting to set up both of them to run in their native resolutions, but am having no luck.

I am following the instructions set out at: http://ubuntuforums.org/archive/index.php/t-1689514.html

But I cannot get past the step where I add a new monitor mode, ie:

xrandr --addmode VBOX1 1600x1200_60.00 as I get an error message:

X Error of failed request: BadMatch (invalid parameter attributes)
Major opcode of failed request: 151 (RANDR)
Minor opcode of failed request: 18 (RRAddOutputMode)
Serial number of failed request: 20
Current serial number in output stream: 21

I have gone through a number of forums, installed the latest version of VirtualBox4, and installed the Guest OS Runtime utilities.

I have also made sure that my virtual display can handle this, ie: xrandr | grep -i maximum

yields:

minimum 64 x 64, current 800 x 600, maximum 32000 x 32000

Has anyone else encountered something similar?