I've run six plasmas before at 1280x720 on a single Catalyst using two TH2G boxes. I was also using The nVidia Quatro FX 5600 Video card. The application was an interactive kiosk which ran cues triggered from iCal. The whole thing was tested and rehearsed before ever going online. I ran seven layers with an Mtron SSD. The system was stable enough for the application but nothing I would rely on in a show environment.

If you want to run six screens at a resolution higher than 1280 rez, then I suggest three Catalyst machines using dual outputs only. In worse case scenario, run two boxes with a DH2G on one output and direct on the second.

Ultimately though Richard has the best point. If you use DV codecs and standard DV size and no one would notice unless they are right in front of the screen.