I'm using the nv driver, and can only get a maximum 800x600 resolution by
default as well. What worked for me was adding the following lines to my
/etc/X11/xorg.conf file (be sure to make a backup first, just in case).
Section "Monitor"
Identifier "Configured Monitor"
HorizSync 30-61
VertRefresh 50-120
EndSection
HorizSync and VertRefresh are the horizontal and vertical refresh rates for
your monitor. Your values will probably be different. You can find them in
the manual that came with your monitor, or, if you don't have that, you
should be able to find them online.
