  Using coherent-potential to describe disorder, Gaspari-Gyorffy approach to
evaluate electron-phonon coupling, and Allen-Dynes equation to calculate T_c,
we show that in Mg_{1-x}M_{x}B_{2} (M = Al,Li or Zn) alloys (i) the way T_c
changes depends on the location of the added/modified k-resolved states on the
Fermi surface and (ii) the variation of T_c as a function of concentration is
dictated by the B p DOS. In addition, using full-potential calculations for
MgMB_4, we show that (i) at x=0.5 a superstructure can form in
Mg_{1-x}Al_{x}B_2 but not in Mg_{1-x}Li_{x}B_2 or Mg_{1-x}Zn_{x}B_2, and (ii) B
layer shifts towards the impurity layer, more for Al than for Li or Zn.
