  By an accurate Green's function method we calculate conductances and the
corresponding Giant Magneto-Resistance effects (GMR) of two metallic
ferromagnetic films separated by different spacers, metallic and non-metallic
ones, in a simplified model on a sc lattice, in CPP and CIP geometries (i.e.
current perpendicular or parallel to the planes), without impurities, or with
interface- or bulk impurities. The electronic structure of the systems is
approximated by two hybridized orbitals per atom, to mimic s-bands and d-bands
and their hybridization.
  We show that such calculations usually give rough estimates only, but of the
correct order of magnitude; in particular, the predictions on the impurity
effects depend strongly on the model parameters. One of our main results is the
prediction of huge CPP-GMR effects for {\it non-metallic} spacers in the
ballistic limit.
