A soft-spin Heisenberg model is used to investigate the Curie temperature of elementary iron and iron-rich intermetallics such as Sm₂Fe₁₇. In the localized limit the mean-field solution of the classical Heisenberg model is reproduced, while quasi-localized perturbation theory accounts for band-structure effects. An extrapolation of this quasi-localized approach yields physically reasonable results, which makes it possible to interpolate between the delocalized band-structure and localized Heisenberg limits. Using Stoner-type magnetic energies derived from paramagnetic band-structure calculations it is argued that the comparatively low Curie temperature of iron-rich rare-earth intermetallics such as R₂Fe₁₇ originates from their quasi-weak ferromagnetism. Upon interstitial modification, these materials leave the quasi-weak limit and become nearly strong ferromagnets with enhanced Curie temperature.