Re: export DISPLAYRESOLUTION
To confuse you there are two things.
Originally Xinerama was an extension for X which made it possible to combine multiple videocards in one 'big desktop'. The Xinerama extension doesn't require you to use monitors of the same dimensions. In case monitors of different sizes are mixed a virtual resolution is used on the small monitor. (For instance if you use two monitors and one uses 1024x768 and the other 800x600, the second screen for instance gets a visible resolution of 800x600 and a virtual one of 1024x768) The xinerama extension provides allows applications to query what the visible resolution of the active display is.
When you enable Twinview support of the nvidia drivers you get a big desktop (unless you use Clone mode). For compatibility with programs, Nvidia's Twinview emulates the Xinerama extension. To make it even more complicated you can also use seperate X screens and enable xinerama on that. In that case the Xinerama extension from X is used whereas in case of Twinview Nvidia's Xinerama emulation is used.