As in title, I want to use my notebook's external monitor with a resolution different from the notebook's primary monitor.

The reason is that I have a widescreen monitor (1680x1050) and therefore switching to a 4:3 resolution in order to use an external monitor brought the "virtual desktop" effect.

I use xfce4, Nvidia GeForceGO5200, Gentoo 2004.3, with latest driver and latest kernel.
The notebook is a Dell Inspiron 8600.

How can I do that? Councils? Howtos? RTFMs?

Since you don't have a distro posted, I'll use mine as an example. There are a couple of ways. Since your setup is a bit more dynamic, being a laptop and all, I think the key combo is ALT+CTRL+PLUS, that increases the res of the screen, and if you use the - key that decreases. I don't know as I've never tried to dynamically change my screen res, but the resolutions that are chosen are predicated via your xorg.conf file. Likewise, since I've never used those key combos, I don't know if having your mouse on one screen will change it just for that screen, as I've never done it.

Now, to your xorg.con file, it's in /etc/X11, it may be called something like XF86config or something like that as well.

In there you should have a section that looks like this ( Please, note the words "looks like", this doesn't mean exactly ):

Section "Screen"
Identifier "Screen1"
Device "Videocard1"
Monitor "Monitor1"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1280x1024"
EndSubSection
EndSection

The part you should be interested in is the Modes. That's where you set your res. For a dual monitor setup you should have two "Screen" sections, one is your main, one is your other. You'll have to use your gray matter to figure it out, but once you change it to what you want, then just restart your windowed environment.

Here is my monitor section (crt + tv)

# **********************************************************************
# Monitor section
# **********************************************************************

# Any number of monitor sections may be present

Section "Monitor"

Identifier "CRT-0"

# HorizSync is in kHz unless units are specified.
# HorizSync may be a comma separated list of discrete values, or a
# comma separated list of ranges of values.
# NOTE: THE VALUES HERE ARE EXAMPLES ONLY. REFER TO YOUR MONITOR'S
# USER MANUAL FOR THE CORRECT NUMBERS.

HorizSync 27 - 115

# HorizSync 30-64 # multisync
# HorizSync 31.5, 35.2 # multiple fixed sync frequencies
# HorizSync 15-25, 30-50 # multiple ranges of sync frequencies

# VertRefresh is in Hz unless units are specified.
# VertRefresh may be a comma separated list of discrete values, or a
# comma separated list of ranges of values.
# NOTE: THE VALUES HERE ARE EXAMPLES ONLY. REFER TO YOUR MONITOR'S
# USER MANUAL FOR THE CORRECT NUMBERS.

VertRefresh 80 - 160
Option "DPMS"

EndSection


Section "Monitor"
Identifier "TV-0"
HorizSync 30 - 50
VertRefresh 75
EndSection

Card:
**********************************************************************
# Graphics device section
# **********************************************************************

# Any number of graphics device sections may be present

Section "Device"
Identifier "MX460"
Driver "nvidia"
Option "nologo" "1"
Option "CursorShadow" "0"
BusId "PCI:1:0:0"
Screen 0
#VideoRam 65536
# Insert Clocks lines here if appropriate

# Option "TwinView"
# Option "ConnectedMonitor" "CRT-0, TV-0"
# Option "TwinViewOrientation" "Clone"
# Option "SecondMonitorHorizSync" "10 - 60"
# Option "SecondMonitorVertRefresh" "50 - 60"
# Option "MetaModes" "1280x1024,;1280x960,;1024x768,;800x600,;640x480,;,400x3
00"
# Option "TVStandard" "PAL-I"
# Option "TVOutFormat" "COMPOSITE"
# Option "TvOverscan" "0.7"

EndSection

Section "Device"
Identifier "MX460TV"
Driver "nvidia"
Option "nologo" "0"
Option "CursorShadow" "0"
BusId "PCI:1:0:0"
Screen 1
Option "TVStandard" "PAL-I"
Option "TVOutFormat" "COMPOSITE"
Option "TvOverscan" "0.6"

EndSection

and screens:
**********************************************************************
# Screen sections
# **********************************************************************

# Any number of screen sections may be present. Each describes
# the configuration of a single screen. A single specific screen section
# may be specified from the X server command line with the "-screen"
# option.

Section "Screen"
Identifier "Screen0"
Device "MX460"
Monitor "CRT-0"

# If your card can handle it, a higher default color depth (like 24 or 32)
# is highly recommended.

# DefaultDepth 8
# DefaultDepth 16
DefaultDepth 24
# DefaultDepth 32

# "1024x768" is also a conservative usable default resolution. If you
# have a better monitor, feel free to try resolutions such as
# "1152x864", "1280x1024", "1600x1200", and "1800x1400" (or whatever your
# card/monitor can produce)

Subsection "Display"
Depth 8
Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 16
Modes "1280x1024" "1280x960" "1152x864" "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 24
Modes "1280x1024" "1280x960" "1152x864" "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 32
Modes "1280x1024" "1280x960" "1152x864" "1024x768" "800x600" "640x480"
EndSubsection
EndSection

Section "Screen"
Identifier "Screen1"
Device "MX460TV"
Monitor "TV-0"
DefaultDepth 24
Subsection "Display"
Depth 24
Modes "640x480" "400x300"
EndSubsection
EndSection

At the moment I am trying to separate them with a mouse each so this section needs a little more tinkering!!

**********************************************************************
# ServerLayout sections.
# **********************************************************************

# Any number of ServerLayout sections may be present. Each describes
# the way multiple screens are organised. A specific ServerLayout
# section may be specified from the X server command line with the
# "-layout" option. In the absence of this, the first section is used.
# When now ServerLayout section is present, the first Screen section
# is used alone.

Section "ServerLayout"

# The Identifier line must be present
Identifier "Simple Layout"

# Each Screen line specifies a Screen section name, and optionally
# the relative position of other screens. The four names after
# primary screen name are the screens to the top, bottom, left and right
# of the primary screen. In this example, screen 2 is located to the
# right of screen 1.

Screen 0 "Screen0"
Screen 1 "Screen1" #rightOf "Screen0"

# Each InputDevice line specifies an InputDevice section name and
# optionally some options to specify the way the device is to be
# used. Those options include "CorePointer", "CoreKeyboard" and
# "SendCoreEvents".

InputDevice "Mouse0" "CorePointer"
InputDevice "Mouse1" "CorePointer"
InputDevice "Keyboard1" "CoreKeyboard"

EndSection