I can only choose 640x480

You should try to run dpkg-reconfigure xserver-xorg, where you should choose the configuration that best matches your hardware.

Once you have completed this step, launch dexconf in order to update x.org configuration file and restart X. Now you should be able to change video resolution correctly.

I've tried it yesterday and it didn't work... And the i810resolution doesn't help neither. :S

(I mean 915resolution :P)

Maybe you should try to use xvidtune in order to adjust screen properties.

I've it installed (xvidtune) but I don't know how to use it.

And i've searched about it and everywhere there says something about a file named XF86Config thats in /etc/X11 but I don't have it :S

Ubuntu (and most other people) use a different version of X, you can find the equivalent of XF86Config at /etc/X11/xorg.conf

Maybe https://help.ubuntu.com/community/FixVideoResolutionHowto will be useful to you.

Ah, ok. Because everywhere I saw 'XF86Config' I thinked its another file :p

I've done what that page says but I don't help :(

I've changed the video memory in the bios from 1MB to 8MB, and:

- The good new: Now it should work at 1024x768 (at least, when I start it the screen says it's 1024x768)

- The bad new: I can't see anything because the screen says that the video frequence is too high. That problem started before change the bios, when I was playing with the xorg.conf trying to get 1024x768 at 8bytes or something less :P but i don't know how to correct it :S I've restaured an xorg.conf from some days ago but it still says that the video frequence is too high. What's the problem (vertical and horitzontal refresh are correct, I think)?

It works!!! :D :D :D :D

Could you please attach your xorg.conf file?

Here you have it, but that wasn't the problem (video memory in bios was too low):

_______________________________________________________________

# /etc/X11/xorg.conf (xorg X Window System server configuration file)
# Help page: "man /etc/X11/xorg.conf"
# Update file: sudo dpkg-reconfigure -phigh xserver-xorg

Section "Files"
 FontPath "/usr/share/X11/fonts/misc"
 FontPath "/usr/share/X11/fonts/cyrillic"
 FontPath "/usr/share/X11/fonts/100dpi/:unscaled"
 FontPath "/usr/share/X11/fonts/75dpi/:unscaled"
 FontPath "/usr/share/X11/fonts/Type1"
 FontPath "/usr/share/X11/fonts/100dpi"
 FontPath "/usr/share/X11/fonts/75dpi"
 # path to defoma fonts
 FontPath "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
EndSection

Section "Module"
 Load "bitmap"
 Load "ddc"
 Load "dri"
 Load "extmod"
 Load "freetype"
 Load "glx"
 Load "int10"
 Load "type1"
 Load "vbe"
EndSection

Section "InputDevice"
 Identifier "Generic Keyboard"
 Driver "kbd"
 Option "CoreKeyboard"
 Option "XkbRules" "xorg"
 Option "XkbModel" "pc104"
 Option "XkbLayout" "es"
EndSection

Section "InputDevice"
 Identifier "Configured Mouse"
 Driver "mouse"
 Option "CorePointer"
 Option "Device" "/dev/input/mice"
 Option "Protocol" "ExplorerPS/2"
 Option "ZAxisMapping" "4 5"
 Option "Emulate3Buttons" "true"
EndSection

Section "Device"
 Identifier "Intel Corporation 82845G/GL"
 #Driver "vesa"
 Driver "i810"
 BusID "PCI:0:2:0"
EndSection

Section "Monitor"
 Identifier "L50A"
 Option "DPMS"
 HorizSync 31-60
 VertRefresh 56-75
EndSection

Section "Screen"
 Identifier "Default Screen"
 Device "Intel Corporation 82845G/GL"
 Monitor "L50A"
 DefaultDepth 16
 SubSection "Display"
  Depth 1
  Modes "1024x768" "800x600"
 EndSubSection
 SubSection "Display"
  Depth 4
  Modes "1024x768" "800x600"
 EndSubSection
 SubSection "Display"
  Depth 8
  Modes "1024x768" "800x600"
 EndSubSection
 SubSection "Display"
  Depth 15
  Modes "1024x768" "800x600"
 EndSubSection
 SubSection "Display"
  Depth 16
  Modes "1024x768" "800x600" "640x480"
 EndSubSection
 SubSection "Display"
  Depth 24
  Modes "1024x768" "800x600"
 EndSubSection
EndSection

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

Section "DRI"
 Mode 0666
EndSection
_______________________________________________________________

Well it's a bit long since this was closed, but here is a screenshot of what should be changed to fix it, for the case someone has the same problem: http://img504.imageshack.us/img504/3893/video8mbvi8.jpg