I don't claim to be an expert but I've configured a few systems using dual monitors. Firstly make sure your card supports twinview - I think you should be OK here.
Next configure your /etc/X11/XF86config-4 file to tell the X server you want to enable two screen support. See the sample XF86config-4 configuration file supplied with the nvidia drivers on how to do this or see the very long README file from nvidia. There is a bit of trial and error in this stage and if your default runlevel is set to 5 you may get into a situation. I set mine to 3 and start the Xserver with startx until I know it's all working then I change the default run level to 5 (in /etc/inittab)
How this helps