You can usually fix resolution problems such as this by running the following command:

sudo dpkg-reconfigure -phigh xserver-xorg

Run that in a terminal and it will walk you through (re)configuring your X server with additional resolutions. If you still don't have that resolution after going through the reconfig try editing your xorg.conf file by hand, adding the resolution you want:

sudo gedit /etc/X11/xorg.conf

...and add "1440x900" as the first resolution under "Section Screen"->"Subsection Display". Example:

SubSection "Display"
  Depth  24
  Modes  "1440x900" "1024x768"
EndSubSection

Make sure to add it under the depth you have set as the default (i.e. what "DefaultDepth" is set to in that same section of the file). Any other resolutions you have there will be available to applications in X (say, full-screen games) but you can cycle through all resolutions manually by typing ctrl-+ at any time in X. You should also be able to change them using whatever GUI you want (say, the "Screen Resolution" tool under the control panel in Gnome).

-Riskable
http://riskable.com
"I have a license to kill -9"

Thanks for your advices... i will keep that in mind.
But I managed it on my own 2 hours ago :-)

I finally succeded in installing the propriate nvidia-driver - took an horrible amount of time, but i made it. Now I can set the solution to 1440x900. As far as I'm testing it now, it seems to work.

Nevertheless, thank you very much.

Greetings
Jörg