Re: GeForce FX 5600XT, wrong resolution options.
Open the Nvidia X Server Settings tool...
Select the XServer Display Configuration and
make sure you can see your external monitor - click on the button 'Detect Displays' if you can't.
If you can see it but it is identified only as 'CRT-0' or something similar without showing a manufacturer or model then...
Under the GPU section select 'CRT-0' (or whatever your monitor is listed as - I think it depends on the connection)
You will now see on the right two items - 'Refresh rate' which is probably unknown and a button 'Acquire EDID' - press that button and follow prompts to save the file.
Now when you go back to the displays, possibly having to press 'Detect Displays' again you should see the model or manufacturer and the correct resolution.
EDID stands for 'Extended Display Identification Data'