which ati card you exactly have?
i heard that the 5870 chipset with 6 display port doesn't work with all outputs at once!
tested a ati 6970 with 2xdvi and 4xdisplay port, and it worked...
we also connected 2 matrox triple heads for extra outputs, so we hat 9 outputs...
but the performance was quiet lame, we hat to set the resolution to 640x480 on every screen in windows to get a stable framerate!
i use a gtx 285 combined with 9600 and 9500 cards for extra outputs which performs better...
had 8 outputs @ 1024x768 (at my nvidia card it doesn't seem to matter which monitor resolition i use) and i could run about 6 layers with framerate over 25fps!
don't know why the older gtx 285 seems to perform better, even with extra graphic cards...
my suggestion is the 512bit memory interface... the only point where the gtx 285 is better than the ati...
also may the drivers/cards from nvidia are better for massive multiscreening
also the graphic memory is smaller on my nvidia card compared to the 6970!
the tests were in resolume 3, with the 6970 we started the beta of arena to try a 8192x768 composition and the performance was much slower than in r3 with 4096x1536! don't know if it's better now in the final version!
maybe the new 7970 can handle 12 monitors with good resolution and performance (384 bit)
beside this card i would suggest the gtx 580 which has also 384 bit...
the new gtx 680 has only 256bit and i bet this card is slower in resolume than the gtx 580
the best solution should be using 2 computers... will work for sure and you can sync it via osc/midi
i bet a gtx 580 with extra nvidia 520 cards and matrox triple heads to get the output you want should be the fastest solution for a single solution!