  We investigate the thermodynamics and signatures of a polariton condensate
over a range of densities, using a model of microcavity polaritons with
internal structure. We determine a phase diagram for this system including
fluctuation corrections to the mean-field theory. At low densities the
condensation temperature, T_c, behaves like that for point bosons. At higher
densities, when T_c approaches the Rabi splitting, T_c deviates from the form
for point bosons, and instead approaches the result of a BCS-like mean-field
theory. This crossover occurs at densities much less than the Mott density. We
show that current experiments are in a density range where the phase boundary
is described by the BCS-like mean-field boundary. We investigate the influence
of inhomogeneous broadening and detuning of excitons on the phase diagram.
