SLI is not just about 'running' an application. SLI offers greater antialiasing and anisotropic filtering which are important when you are using multi-monitors. SLI does make smoother pictures in portrait mode versus landscape, but some of that is because of perspective skewing (stretching). If you are familiar with how area calculations are done you can see that the "scalable" term comes into play when working with anisotropic filtering because the amount of filtering available from a single card does not extend the filtering 'range' much further from the users' viewpoint, because the area is so much greater for each increase in view range. That is the downside to increasing your view extents (number of monitors) because as your antialiasing requirements increase your ability to process filtering decreases. You also need to be aware that there is a limit to movie making, because codecs do not exist that are able to handle the bit-rates required for three monitor resolutions in streaming videos. So, probably portrait mode would work better for videos, but it will put your TrackIR pickup above your head level (if you use one). Orientation is an individual choice, of course, and depending on how else you use your system will determine your choice.
The problem with scaling is that there is a limitation to the lengths you can go to, because you are always limited by the available memory on the first card. This will be changing in the near future.
