After installing your ubuntu you might face the problem that 800x600 is the maximum resolution of your screen.
make a backup of "/etc/X11/xorg.conf"
copy/paste this code in /etc/X11/xorg.conf logout and login again then check your modes
This xorg.conf is for VESA driver (supports most unsupported cards)

Section "Files"

• FontPath "/usr/share/fonts/misc:unscaled"
  FontPath "/usr/share/fonts/local"
  FontPath "/usr/share/fonts/75dpi:unscaled"
  FontPath "/usr/share/fonts/100dpi:unscaled"
  FontPath "/usr/share/fonts/Type1"
  FontPath "/usr/share/fonts/URW"
  FontPath "/usr/share/fonts/Speedo"
  FontPath "/usr/share/fonts/PEX"
  FontPath "/usr/share/fonts/cyrillic"
  FontPath "/usr/share/fonts/latin2/misc:unscaled"
  FontPath "/usr/share/fonts/latin2/75dpi:unscaled"
  FontPath "/usr/share/fonts/latin2/100dpi:unscaled"
  FontPath "/usr/share/fonts/latin2/Type1"
  FontPath "/usr/share/fonts/latin7/75dpi:unscaled"
  FontPath "/usr/share/fonts/baekmuk:unscaled"
  FontPath "/usr/share/fonts/japanese:unscaled"
  FontPath "/usr/share/fonts/kwintv"
  FontPath "/usr/share/fonts/truetype"
  FontPath "/usr/share/fonts/uni:unscaled"
  FontPath "/usr/share/fonts/CID"
  FontPath "/usr/share/fonts/ucs/misc:unscaled"
  FontPath "/usr/share/fonts/ucs/75dpi:unscaled"
  FontPath "/usr/share/fonts/ucs/100dpi:unscaled"
  FontPath "/usr/share/fonts/hellas/misc:unscaled"
  FontPath "/usr/share/fonts/hellas/75dpi:unscaled"
  FontPath "/usr/share/fonts/hellas/100dpi:unscaled"
  FontPath "/usr/share/fonts/hellas/Type1"
  FontPath "/usr/share/fonts/misc/sgi:unscaled"
  FontPath "/usr/share/fonts/xtest"
  FontPath "/opt/kde3/share/fonts"
  InputDevices "/dev/gpmdata"
  InputDevices "/dev/input/mice"
  

EndSection

Section "ServerFlags"

• Option "AllowMouseOpenFail" "on"
  Option "ZapWarning" "on"
  

EndSection

Section "Module"

• Load "dbe"
  Load "type1"
  Load "freetype"
  Load "extmod"
  

EndSection

Section "InputDevice"

• Driver "kbd"
  Identifier "Keyboard[0]"
  Option "Protocol" "Standard"
  Option "XkbLayout" "be" keyboard layout, edit this stuff and remove my comment
  Option "XkbModel" "microsoftpro"
  Option "XkbOptions" "caps:shiftlock"
  Option "XkbRules" "xfree86"
  

EndSection

Section "InputDevice"

• Driver "mouse"
  Identifier "Mouse[1]"
  Option "Buttons" "5"
  Option "Device" "/dev/input/mice"
  Option "Name" "ImPS/2 Generic Wheel Mouse"
  Option "Protocol" "explorerps/2"
  Option "Vendor" "Sysp"
  Option "ZAxisMapping" "4 5"
  

EndSection

Section "Monitor"

• Option "CalcAlgorithm" "XServerPool"
  DisplaySize 338 270
  HorizSync 30-83
  Identifier "Monitor[0]"
  ModelName "HP HP F1703"
  Option "DPMS"
  Option "PreferredMode" "1280x1024"
  VendorName "HWP"
  VertRefresh 43-76
  UseModes "Modes[0]"
  

EndSection<<BR>

Section "Modes"

• Identifier "Modes[0]"
  Modeline "1280x1024" 108 1280 1328 1440 1688 1024 1025 1028 1066 +hsync +vsync
  

EndSection

Section "Screen"

• DefaultDepth 24
  SubSection "Display"
  

  • Depth 15
    Modes "1280x1024" "1024x768" "800x600" "768x576"
    

  EndSubSection
  SubSection "Display"
  

  • Depth 16
    Modes "1280x1024" "1024x768" "800x600" "768x576"
    

  EndSubSection
  SubSection "Display"
  

  • Depth 24
    Modes "1280x1024" "1024x768" "800x600" "768x576"
    

  EndSubSection
  SubSection "Display"
  

  • Depth 8
    Modes "1280x1024" "1024x768" "800x600" "768x576"
    

  EndSubSection
  Device "Device[0]"
  Identifier "Screen[0]"
  Monitor "Monitor[0]"
  

EndSection

Section "Device"

• BoardName "some vesa board"
  BusID "1:0:0" may cause problems, use lspci to check the bus in your system
  Driver "vesa"
  Identifier "Device[0]"
  

EndSection

Section "ServerLayout"

• Identifier "Layout[all]"
  InputDevice "Keyboard[0]" "CoreKeyboard"
  InputDevice "Mouse[1]" "CorePointer"
  Option "Clone" "off"
  Option "Xinerama" "off"
  Screen "Screen[0]"
  

EndSection

Section "DRI"

• Group "video"
  Mode 0660
  

EndSection

Section "Extensions"

• Option "Composite" "off"
  

EndSection