It's things like mini display port that prove Apple gets too high on their horse and makes stupid decisions. I have tested the mini display port on the 4870 and all it seems to be good for is running a second 30" cinema display. It doesn't like the TH2G pr DH2G boxes that well. Yes, I use the Dual Link box which is a pain in the butt.

In short, I suggest three ways for your show in the order I would choose.

The first way, the way I would do it is to use the DVI output of two Mac Pros with 4870 cards, attached to a TH2G each set for 1280x720x3. Your monitors will only see 1280x720 but the chances of anyone noticing you're not pixel accurate is unlikely.

The second solution is to use six Mac Minis (three per tower). I like this idea because it puts the Media server right at the device so ethernet is all you need run.

The third solution is to use three Mac Pros running two outputs each.

Solution 1 is best for perfectly synchronous content playback between adjacent monitors. Solution 2 is simplest and has smallest footprint. Solution 3 is the one which will allow the greatest number of layers to be run per output.