I went through a night or two of hell with this issue until finally I got it working..and I'm not entirely sure how. I started off doing what was advised in the first message, adding the modes, whatever that did. It didn't seem to immediately fix anything, but as far as I know it might have been part of what fixed the problem later.

I also did xorgconfig, or whatever it's called, and even tried manually editing /etc/X11/xorg.conf. Nothing. I was stuck in a low terrible resolution.

I later was advised to do: sudo dpkg-reconfigure xserver-org, which I did several times to no avail.

At some point, after doing various combinations of these procedures many many times, I went into xorg.conf, and found that my horizontal and vertical rates were not defined. I defined them, and instantly it all worked. Here is my xorg.conf if this helps anyone:

# /etc/X11/xorg.conf (xorg X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the /etc/X11/xorg.conf manual page.
# (Type "man /etc/X11/xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following commands:
#
# cp /etc/X11/xorg.conf /etc/X11/xorg.conf.custom
# sudo sh -c 'md5sum /etc/X11/xorg.conf >/var/lib/xfree86/xorg.conf.md5sum'
# sudo dpkg-reconfigure xserver-xorg

Section "Files"
FontPath "unix/:7100" # local font server
# if the local font server has problems, we can fall back on these
FontPath "/usr/lib/X11/fonts/misc"
FontPath "/usr/lib/X11/fonts/cyrillic"
FontPath "/usr/lib/X11/fonts/100dpi/:unscaled"
FontPath "/usr/lib/X11/fonts/75dpi/:unscaled"
FontPath "/usr/lib/X11/fonts/Type1"
FontPath "/usr/lib/X11/fonts/CID"
FontPath "/usr/lib/X11/fonts/100dpi"
FontPath "/usr/lib/X11/fonts/75dpi"
# paths to defoma fonts
FontPath "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
FontPath "/var/lib/defoma/x-ttcidfont-conf.d/dirs/CID"
EndSection

Section "Module"
Load "GLcore"
Load "bitmap"
Load "dbe"
Load "ddc"
Load "dri"
Load "extmod"
Load "freetype"
Load "glx"
Load "int10"
Load "record"
Load "type1"
Load "v4l"
Load "vbe"
EndSection

Section "InputDevice"
Identifier "Generic Keyboard"
Driver "keyboard"
Option "CoreKeyboard"
Option "XkbRules" "xorg"
Option "XkbModel" "pc105"
Option "XkbLayout" "dvorak"
EndSection

Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
Option "CorePointer"
Option "Device" "/dev/input/mice"
Option "Protocol" "ImPS/2"
Option "ZAxisMapping" "4 5"
EndSection

Section "Device"
Identifier "Intel Corporation 82865G Integrated Graphics Device"
Driver "i810"
BusID "PCI:0:2:0"
VideoRam 64000
EndSection

Section "Monitor"
Identifier "Acer AL1715"
Option "DPMS"
HorizSync 31.5-80
VertRefresh 36.5-75
EndSection

Section "Screen"
Identifier "Default Screen"
Device "Intel Corporation 82865G Integrated Graphics Device"
Monitor "Acer AL1715"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1280x1024" "1152x864" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x1024" "1152x864" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024" "1152x864" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x1024" "1152x864" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024" "1152x864" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024" "1152x864" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection
EndSection

Section "ServerLayout"
Identifier "Default Layout"
Screen "Default Screen"
InputDevice "Generic Keyboard"
InputDevice "Configured Mouse"
EndSection

Section "DRI"
Mode 0666
EndSection