256 GPUs.... I think you've lost sight of another factor... There is a reason I only have 2 6990s in my rig, cost.
but not everybody has your limited wallet size...
think of the money yuo would save...
i.e running windows...
for every 8 GPUs you need another motherboard, CPU, ram, harddrive.
that is the one shot cost...
long term cost is the power needed to supply each MB, CPU .....
if you could get a 256 GPUs running on a single machine it would save $