  Recent hydrodynamic (HD) simulations have shown that galactic disks evolve to
reach well-defined statistical equilibrium states. The star formation rate
(SFR) self-regulates until energy injection by star formation feedback balances
dissipation and cooling in the interstellar medium (ISM), and provides vertical
pressure support to balance gravity. In this paper, we extend our previous
models to allow for a range of initial magnetic field strengths and
configurations, utilizing three-dimensional, magnetohydrodynamic (MHD)
simulations. We show that a quasi-steady equilibrium state is established as
rapidly for MHD as for HD models unless the initial magnetic field is very
strong or very weak, which requires more time to reach saturation. Remarkably,
models with initial magnetic energy varying by two orders of magnitude approach
the same asymptotic state. In the fully saturated state of the fiducial model,
the integrated energy proportions E_kin:E_th:E_mag,t:E_mag,o are
0.35:0.39:0.15:0.11, while the proportions of midplane support
P_turb:P_th:\Pi_mag,t:\Pi_mag,o are 0.49:0.18:0.18:0.15. Vertical profiles of
total effective pressure satisfy vertical dynamical equilibrium with the total
gas weight at all heights. We measure the "feedback yields"
\eta_c=P_c/\Sigma_SFR (in suitable units) for each pressure component, finding
that \eta_turb~4 and \eta_th~1 are the same for MHD as in previous HD
simulations, and \eta_mag,t~1. These yields can be used to predict the
equilibrium SFR for a local region in a galaxy based on its observed gas and
stellar surface densities and velocity dispersions. As the ISM weight (or
dynamical equilibrium pressure) is fixed, an increase in $\eta$ from turbulent
magnetic fields reduces the predicted \Sigma_SFR by ~25% relative to the HD
case.
