Ah - that clarifies it a little more....
We did a similar project (with Ian H) using 7x MacMinis with 1080p outouts to 7x 65in plasmas. The content was taken from a red camera and then cropped vertically to fit the aspect ratio of the 7x 1920x1080 screens. To fit the screens we used a scale of aprox 3x in catalyst to fit the 4480pixel width to 7x 1920 (13440)
screen area. Much more than 3x scale started to pixilate slightly with out setup.
However - this demonstrated that you DONT have to use 1920x1080 1:1 pixel content. Lower resolution will usually look fine.
To achieve the full panorama over 7 Mac-Mini servers, we used the X-offset function on each server to offset the image the required amount on each screen.
For your setup - if you want to use MacPros - then you can use either 2x DH2G - one on each output. This can sometimes cause problems and also loses your programming/setup screen.
Personally - I'd use 1x Datapath X4 on each MacPro, then this also leaves the 1st output as your programming / setup & desktop screen - which you will need to setup mix windows and blending etc. The Datapath also has more control and offers more refresh rate possibilities.
If you are using 2x servers - each with 4x projectors connected. So there is a maximum of 4x HD movie playback, and then crossfade to another 4x - so only a total of 8x movies maximum...
The maximum movie size catalyst will read is currently 4480x2600
you really don't want - or need - to be using files bigger than this... As explained above - smaller movies scales up are sufficient.
The limit is within catalyst - but can be changed - but there is also a limit with your graphics card, and this will depend on the graphics card specs.
The X/Y does not limit playback - only position.
Synchronising 8 movies is no more difficult than synchronising 1x movie - its the same process - so maybe 8x 1920x1080 movies might be an option.
Hope this helps
Do test first!...
Simon