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Setting up dual monitors under Debian with an nVidia graphics card is not as easy as it could be but certainly not beyond the realms of possibility. The problem is that there is not simple configuration system that will do the work for you so you have to modify the X configuration by hand. First of all install the latest nvidia drivers: 

aptitude install nvidia-glx

Then fire up module-assistant and install the nvidia module.  This will down load the required source (for example nvidia-kernel-common, nvidia-kernel-amd64 and the kernel headers) compile the kernel shim for the driver and then install the module. Once this has completed successfully configure the xserver so that it is using the nvidia drivers with a single monitor (a reboot is not strictly necessary but I like to do one to make sure everything comes up cleanly). 

dpkg-reconfigure xserver-xorg

The configuration will ask you numerous questions one of which will be which video card driver you want to use. Choose nvidia rather than the open source nv. If X comes back up using the nVidia drivers (which can easily be spotted by the splash screen) then you are ready to configure your secodn monitor. If the drivers didn't work check that everything is correctly specified and try again.

Open a command prompt and take a copy of your X configuration file.  

su -
cd /etc/X11
cp xorg.conf xorg.conf.working

In the Screen section add the following lines: 

Option "RenderAccel" "true"
Option "TwinView"
Option "MetaModes" "1600x1200 1280x1024"
Option "TwinViewOrientation" "LeftOf" 

The first two lines you should copy verbatim the second two you can / should tweak to your own needs. The MetaModes line specifies the size of the monitors attached. Ideally both monitors will be the same size but mine are different. The TwinViewOrientation line indicates which side the second monitor is on. In my case it is left but full list of optiosn is LeftOf, RightOf, Above, Below or Clone.

Once you are happy with your settings restart your X server with Crtl-Alt-Backspace and enjoy multi-monitor heaven.