Comment 15 for bug 138920

Ubuntu can't detect what resolutions your monitor supports, and the Monitor section of your xorg.conf doesn't list any.

 I would mark this "invalid", but one could consider it a bug that Ubuntu doesn't have a better way to deal with situations where it can't autodetect things. I don't think there's anything wrong with editing a text config file, or using
 sudo dpkg-reconfigure xserver-xorg
but if there is a tool to do it better, Ubuntu should use that.

(The monitor resolution detection stuff is here in your log file:
(II) MGA(0): I2C Monitor info: (nil)
(II) MGA(0): end of I2C Monitor info
(--) MGA(0): No DDC signal

DDC is the signaling method for computers to ask monitors about themselves, etc.

 Since Xorg can't get any info from the monitor, it falls back on very conservative defaults for horizontal and vertical refresh rate limits, which only allow up to 800x600 resolutions. Your log says your vid card has 8MB of RAM, so your card definitely supports 1024x768. If your monitor does too, go look up its specs (hsync and vsync, aka horizontal and vertical refresh range, or whatever.)

on my system, my Xorg.0.log has lines like
...
(II) intel(0): Printing DDC gathered Modelines:
(II) intel(0): Modeline "1680x1050"x0.0 146.25 1680 1784 1960 2240 1050 1053 1059 1089 -hsync +vsync (65.3 kHz)
(II) intel(0): Modeline "800x600"x0.0 40.00 800 840 968 1056 600 601 605 628 +hsync +vsync (37.9 kHz)
...

 Here's an example "Monitor" section:

Section "Monitor"
        Identifier "opti"
        Option "DPMS"
        DisplaySize 320 240
        # 32x24 cm
        HorizSync 30-70
        VertRefresh 50-120
EndSection

HorizSync is in khz, VertRefresh is in Hz.

google for more info if you need it. It's only in the last couple years that you didn't always have to put your refresh rates in your xorg.conf (or previously /etc/X11/XF86Config). There is a ton of old and new info about configuring X...