  We investigate magnetic and superconducting instabilities of the
two-dimensional t-t' Hubbard model on a square lattice at Van Hove densities
from weak to intermediate coupling by means of the two-particle self-consistent
approach. We find that as the next-nearest-neighbor hopping |t'| increases from
zero, the leading instability is towards an incommensurate spin-density wave
whose wave vector moves slowly away from (pi, pi). For intermediate values of
|t'|, the leading instability is towards d_{x2-y2}-wave superconductivity. For
larger |t'|>0.33t, there are signs of a crossover to ferromagnetism at
extremely low temperatures. The suppression of the crossover temperature is
driven by Kanamori screening that strongly renormalizes the effective
interaction and also causes the crossover temperature to depend only weakly on
t'. Electronic self-energy effects for large |t'| lead to considerable
reduction of single-particle spectral weight at zero energy at temperatures as
high as T<0.1t, an effect that may be detrimental to the existence of a
ferromagnetic ground state at weak coupling.
