I had to search and experiment for a while to get two monitors
working on my Linux X11 Gnome / KDE Mandrake 10.1 setup. This post is
just to share how I got it all working in hopes that it helps someone.

First off, this assumes you have X working with a single monitor,
have the basic concepts of X11 configuration, and have setup your
config file using xf86config cmdline utility. One thing this will
do is setup a line in your config with the correct PCI bus info for
your video card.

This issue is mostly independent of the distro. It hardly matters if
you are on RedHat, Suse, Mandrake, etc. and it hardly matters what
desktop or window manager you use, Gnome, KDE, etc. As long as
you have X11 a.k.a X-windows. Its an issue with X11. Of course, first
you need the Nvidia driver.

A) Download the Nvidia driver for linux from http://www.nvidia.com and
follow the instructions to install it. It is provided as a shell
archive.
Run the script as root and it will install the driver as a kernel
module.
I needs one specific for your kernel version. If it doesn't have one it
will try to download one from its ftp site. If that doesn't have one,
it
will try to compile it using your kernel source tree. Lucky for me I
had
recently compiled and installed linux kernel 2.6.10 so my kernel source
tree was where it was supposed to be. This worked great, and it
installed
the driver. You can see the driver installed with the 'lsmod' command.

B) Configuring X11 for two monitors was the last step, but the one that
took the longest to figure out. One way to do this is to run two
desktops,
and two complete copies of the window manager and all the desktop
software.
I didn't like that idea, to much overhead. I wanted one virtual desktop
that spanned monitors. You know, so you can simply drag a window from
one
screen to the other. Rather than bore you with a lot of chat, here is
what
I ended up with that works great:

Below when I refer to config file, I mean /etc/X11/XF86Config

B1) Remove any reference anywhere to Xinerama. The Nvidia drivers make
both monitors look like one big virtual monitor, so the X11 Xinerama
option will just confuse things. In your config file, check the
"ServerFlags" section and remove or comment out Xinerama if its there.
Also check your ~/.xinitrc file and remove any reference to Xinerama.

B2) In the "Monitor" section of the config file, remove the modelines.
You do still need the critical specs of, Identifier, HorizSync,
VertRefresh.
The driver will figure the modes on its own given this info. You should
name the Identifier "CRT-0" since the nvidia driver seems to look for
that
tag name specifically.

B3) Create a second Monitor section that matches your second monitor
specs. If its the same, just copy the previous monitor section, and
rename the Identifier "CRT-1". Yes, the name seems to matter!

For an example, my first one looked like this:

Section "Monitor"
Identifier "CRT-0"
VendorName "Plug'n Play"
ModelName "Dell M990"
HorizSync 30-96
VertRefresh 50-160
EndSection

B4) Fill in the "Device" section. CAUTION, use xf86config or some
other tool to get the correct PCI bus info for your card. Here is
my secton:

Section "Device"
# see /usr/share/doc/NVIDIA* for the readme
Identifier "DeviceTwinView"
VendorName "nVidia Corporation"
BoardName "Unknown Board"
Driver "nvidia"
BusID "PCI:5:0:0" # CAUTION, get the right numbers for your
setup!!!
Option "DPMS"
OPTION "TwinView"
Option "MetaModes" "1024x768, 1024x768; 1280x1024, 1280x1024;
1600x1200, 1600x1200;"
Option "SecondMonitorVertRefresh" "50-160"
Option "TwinViewOrientation" "RightOf"
Option "SecondMonitorHorizSync" "30-96"
EndSection


B5) Fill in the "Screen" section starting with this:

Section "Screen"
Identifier "ScreenTwinView"
Device "DeviceTwinView"
Monitor "CRT-0"

Subsection "Display"
Depth 24
#Modes "1024x768" # don't use this. Use Virtual!
Virtual 2048 768
EndSubsection

....other subsections go here...
EndSection

Then of course reference this screen from the "ServerLayout" section.
That's it! The crux of the matter for me was replacing "Modes" with
"Virutal" in the Screen section. Hope this helps.

It's actually quite easy if you follow the xinerama howto.