I agree with what lukejanicke said in his or her answer as far as it goes. However, having done quite a bit of research on this question (see here), I would add this: yes, the writer needs to decide what to say first, then choose the article. But the issue can also be approached from the perspective of the reader: when I read a title, the various choices for the article placement have certain effects on my reading of it. What kinds of effect are these? Indeed, this is actually the more basic question, because presumably the writer's decision will depend on what sort of effects he or she wishes to impart on the readers.
True, the question is complex because much will depend on the context. But the fact that it is a title does provide for certain advantages as far as analysis. After all, the title is presumably the first thing the reader reads of the text, and we can't assume the reader will know anything else about the work at that time except the title. Therefore, we should be able to say something about the effects that the placement of articles has on the interpretation of the title upon first reading. This initial interpretation is subject to revision once the actual text is read, but the initial reading of the title is still of great interest, since it will affect how we approach the main text, at least in the beginning—not to speak of the fact that it will affect our decision whether to read the rest of the text at all. A writer will want to take all of this into account: how the title will be read upon the first encounter, and how it will be reinterpreted as the reader progresses through the text. Clearly, given the information we are given about Dream of Atlantis, we will only be able to do the former—but that's already pretty interesting.
Here I'll deal with a piece of 'low lying fruit': the effect of adding an article in front of Atlantis in the Dream of Atlantis family of titles.
What makes the analysis a bit easier in this case is that the names of individual islands and continents normally don't get an article (except the names of groups of islands like the Scilly Isles or the Orkneys, and except the names of islands that have an of-phrase in them, such as the Isle of Man). The mythical continent of Atlantis is not an exception, and its name is used without an article, see e.g. here.
Therefore, saying the Atlantis would signal that the word is in fact not referring directly to the mythical continent, but probably to something else that's called Atlantis, like a ship. (Interestingly, though the Space Shuttle named Atlantis is usually referred to without the definite article, see e.g. here, sometimes the definite article is used, like here.)
The indefinite article, Dream of an Atlantis, would most naturally cause this to be read as 'Dream of a society with characteristics traditionally attributed to the culture that supposedly inhabited the mythical island of Atlantis'.
And omitting the article altogether leaves it more ambiguous; after all, this is a title, and we can expect the writer to try to do something symbolic. For example, see the discussion of the title of the novel Tropic of Cancer here. The author Henry Miller intended that title to be a wordplay; he does not ultimately intend it to refer to its usual meaning of 'the circle of latitude at 23°26′12.6″ N'. True, it is unlikely any reader would be able to guess the intended meaning of the author's wordplay before reading the book (and, quite frankly, probably most of us wouldn't be able to guess it even after reading it). But that was Miller's intention: to make it a bit mysterious, because the literal meaning would be really strange given what the novel is about. And Miller was super-ambitious as far as the literary impact he wished to have (see e.g. here).
So when we see Dream of Atlantic, we will think that Atlantic probably refers to the mythical continent (and probably more specifically to its utopian civilization). However, we will be ready to revise this opinion if it turns out not to fit the content of the text, and look for some other sort of symbolism.
As far as the word dream, all I'll say is that here opportunities for ambiguity are even greater, since that word can also be interpreted as a verb.