  We study the linear sigma model with O(N) symmetry at nonzero temperature in
the framework of the Cornwall-Jackiw-Tomboulis formalism. Extending the set of
two-particle irreducible diagrams by adding sunset diagrams to the usual
Hartree-Fock contributions, we derive a new approximation scheme which extends
the standard Hartree-Fock approximation by the inclusion of nonzero decay
widths. In this approximation, in-medium modifications of the meson masses as
well as decay and collisional broadening effects are selfconsistently taken
into account. As compared to the standard Hartree-Fock approximation, the
temperature for the chiral symmetry restoring phase transition is lowered by
\~20%. For large temperatures, the spectral densities of the sigma-meson and
the pion become degenerate, as required for the chirally symmetric phase.
