even using the new motherboards with crossfire x or the nvidia equivelent supporting four gpu's it can't be done you would need 2 motherboards to achieve that.
you could certainly use eight monitors with the right kit but i'm not sure how stable that would be i would probably want 2 or 3 computers linked in parrallel doing that with the gpu's mixed in.
and there is driver
support from either Nvidia or Ati they themselves may not support that many monitors.
going to ask now though i'm interested to know if they support it