  We present a comparative, theoretical study of the doping dependence of the
critical temperature $T_C$ of the ferromagnetic insulator-metal transition in
Gd-doped and O-deficient EuO, respectively. The strong $T_C$ enhancement in
Eu$_{1-x}$Gd$_x$O is due to Kondo-like spin fluctuations on the Gd sites, which
are absent in EuO$_{1-x}$. Moreover, we find that the $T_C$ saturation in
Eu$_{1-x}$Gd$_x$O for large $x$ is due to a reduced activation of dopant
electrons into the conduction band, in agreement with experiments, rather than
antiferromagnetic long-range contributions of the RKKY interaction. The results
shed light on possibilities for further increasing $T_C$.
