The resolution of my lubuntu is 640x480 and I can not change it. There's no more options in the screen definitions.

This is my (horrible) graphic card:

01:00.0 VGA compatible controller: Silicon Integrated Systems [SiS] 771/671 PCIE VGA Display Adapter (rev 10)

And this is what I got when I run the xrandr command:

xrandr: Failed to get size of gamma for output default

Screen 0: minimum 640 x 480, current 640 x 480, maximum 640 x 480

default connected primary 640x480+0+0 0mm x 0mm

   640x480        73.0* 

  800x600_60.00 (0x198)   38.2MHz

        h: width   800 start  832 end  912 total 1024 skew    0 clock   37.4KHz

        v: height  600 start  603 end  607 total  624           clock   59.9Hz

  2048x1536_60.00 (0x1da)  267.2MHz

        h: width  2048 start 2208 end 2424 total 2800 skew    0 clock   95.4KHz

        v: height 1536 start 1539 end 1543 total 1592           clock   60.0Hz

  1024x728_60.00 (0x1dc)   63.5MHz

        h: width  1024 start 1072 end 1176 total 1328 skew    0 clock   47.8KHz

        v: height  768 start  771 end  775 total  798           clock   59.9Hz

  1024x768_60.00 (0x1dd)   63.5MHz

        h: width  1024 start 1072 end 1176 total 1328 skew    0 clock   47.8KHz

        v: height  768 start  771 end  775 total  798           clock   59.9Hz

So, I want to have a 1024x768 resolution in this computer and I can't do nothing in this moment. I'm a newbie in Linux and for that reason I need some help. I tried to follow some steps that I saw in some topics but nothing happens.