According to 'xrandr', your current resolution, 1024x768, is the maximum one.
But again, that may be due to some kind of issue (although it can also be due
to a lack of support for your card in the kernel; the component that includes
the drivers and makes the interface between the hardware and the software).
We need to take a look at the logs to know (unless someone here possesses a
card from the same family and can tell).
All logs are centralized in /var/log. This is a path. It means that, from the
root of the file system named "/" (but called "File system" in the left pane
of the graphical file browser Trisquel ships in the default install), you go
through the directory named "var" and then "log". In this way, when I wrote
"/var/log/Xorg.0.log", I actually gave you where to find the file named
"Xorg.0.log".
That is the file you should attach to your post if you want us to take a look
at Xorg's logs. Xorg is the OS component that provides a basic graphical
environment (the so called "window managers" are built upon it). Without it,
you only get command the line interfaces (aka terminals) that you can access
by pressing Ctrl+Alt+F1, Ctrl+Alt+F2, Ctrl+Alt+F3, Ctrl+Alt+F4, Ctrl+Alt+F5
and Ctrl+Alt+F6. Try it! From a terminal, you come back to the graphical
session by pressing Alt+F7.
Tell me if I am too "teachy" (that actually is part of what I do for a
living; although for more scientific topics, and I am always happy to do so
when someone like you seems eager to learn).
