1920x1200 pixels = 2304000 pixels
@ 4 bytes per pixel make that 9216000 bytes yeh?
divide by 1048576 for megabytes and I get the magic number of
8.8 MBytes per frame buffer.
For three screens that gives me 26.37 MBytes for frame store.
Make that 79.10MBytes for a triple buffered configuration without rendering overhead.
Now consider the "alternative" vendor which did provide a frame locked Compiz desktop with frame locked video playback.
nVidia 9800GT 512M driving 2 x 1920x1200
That's 512M driving 2/3 of the pixels the ATI's 1GB is driving.
Going by nVidia's numbers there's the possibly of being able to drive four of those screens with 1GB.
Am I missing something in my numbers?