Haha, yes, you're right, the AI in GalCiv is pretty good compared to Civ's...
The second time I declared war on someone they folded to the already-strongest empire
Sorry to be a party breaker, but i find it hard to call that behaviour "inteligent". Call it "annoying to the human player", challeging to the human player" or something of that kind , but that is NOT inteligent behaviour by the part of that particular AI, especially in Civ IV, where you can win with one city and, barring PA, there is no way of making another player win my own ( as almost all of the GalCiv II wins are, atleast acording to the GalCiv II wiki ( since I never played the game )). So, there is no real incentive to a player to throw himself under the good graces of a protecting power just because it is being beaten to crap: being a vassal of someone who wins does not make you win in Civ IV

That kind of behaviour can make the game more challenging , but that is in the exact oposite side of being inteligent, atleast seen in the prespective of the weaker civ in Civ IV that technically can win until the moment it loses the last city

That said, in civ IV, where every civ can only win by themselfes barring PA, throwing themselfes into the mercy of a friendly civ or wait until vassaling to the agressor have the same value in terms of chances of winning... in fact vassaling to the agressor may even be a better strategy in a lot of cases

This applies also to the argument that the AI should jump to burn UN/AP , the capital or one of the legendary cities to avoid a enemy win : ok, that might make the other guy not win the game, but it will make the attacker win it? Not necessarily ( and, given that those raids sometimes are pretty much
kamikaze attacks, where you don't expect to see your soldiers back, in fact it can even make the player in question more prone to lose the game

)
Other aspect to have in mind is that, unlike it seems to me from reading the victory conditions of GalCiv II, Civ IV victory conditions allow for very sudden jumps in the degree that a civ is close of winning via a certain VC and you can pass easily from having 50% of the win conditions fulfilled to 100% in one turn or even in the same turn for all the victory conditions. As it is easy to understand, gradual wins are far more easy to detect and counter by a AI than things that can sudently jump a lot. if you say to a AI " start preparing the ships to attack a civ when it reaches the second legendary city " ( for the sake of example .. it could be the first engine of the SS, or 40% of the expected votes in UN , or 30% of the land mass .... chose one, the logic behind is the same ), what the AI will do if I get the second legendary city at the same time as the third? How will the AI react to a player ( human or not .... ) that builds all the space parts in the same turn and relocates the capital to a spot inacessible in the turns that are needed to the ship to get to AC ? Or worse, what happens if the AI gets to be ready in time to burn the capital of a civ that built a slow SS just to be surprised by the launch of another SS in the other side of the world?
Bottom line, I strongly believe that the behaviour that people describe as being of GalCiv II AI is not adequate to Civ IV AI due to the diference between the victory rules. And , in another line of thought ( that I already used elsewhere regarding the same issue ), I have my serious doubts that making the life hard to a player that is in the way of winning can be described as inteligent behaviour in a enviroment that, besides one exception ( the already quoted PA ), doesn't allow comunal wins.