On Fri, Oct 2, 2009 at 8:49 AM, Mark Mwangi <mwangy@gmail.com> wrote:

Hi Mwangi, am using gnome so grandr works for me. . . here is my
xrandar output. . .

mwangi@mwangi-desktop:~$ xrandr
Screen 0: minimum 320 x 240, current 800 x 600, maximum 800 x 600
default connected 800x600+0+0 0mm x 0mm
800x600 60.0* 56.0
640x480 60.0
400x300 60.0 56.0
320x240 60.0

So your screen is "default".
And this screen has a max resolution of 800x600. What you need to do is use cvt to generate a valid mode, import this mode into xrandr for 'default' and activate it.
An example:
My vbox instance has the screen VBOX1

--
Get the current modes
--
lmwangi@hyperion:~$xrandr
Screen 0: minimum 64 x 64, current 800 x 600, maximum 32000 x 32000
VBOX1 connected 800x600+0+0 0mm x 0mm
   800x600        60.0 +   60.3*
   640x480        59.9
--
Generate a new mode
--
lmwangi@hyperion:~$cvt 1280 800
# 1280x800 59.81 Hz (CVT 1.02MA) hsync: 49.70 kHz; pclk: 83.50 MHz
Modeline "1280x800_60.00"   83.50 1280 1352 1480 1680 800 803 809 831 -hsync +vsync

--
Add it to xrandr
--

lmwangi@hyperion:~$xrandr --newmode "1280x800_60.00"   83.50 1280 1352 1480 1680 800 803 809 831 -hsync +vsync

lmwangi@hyperion:~$xrandr
Screen 0: minimum 64 x 64, current 800 x 600, maximum 32000 x 32000
VBOX1 connected 800x600+0+0 0mm x 0mm
   800x600        60.0 +   60.3*
   640x480        59.9
1280x800_60.00 (0x4a)   83.5MHz
        h: width 1280 start 1352 end 1480 total 1680 skew    0 clock   49.7KHz
        v: height 800 start 803 end 809 total 831           clock   59.8Hz

--
Add it to the screen
--

lmwangi@hyperion:~$xrandr --addmode VBOX1 1280x800_60.00
--
Activate
--

regards,
laban