I think Civ4 and GalCiv2 are excellent games, which I have played alot.
However, Dom 3 is better, it has far more strategy and depth than either of those. Well basic playing mechanics stays the same, different nations can have completely different strategies.
In terms of tactics, depth and so on Civ4 is like bike while Dom3 is a car. You really cannot compare both of them. Civ4 is for people that think they like strategies. Dominions is for people that really love them.