It depends on the language. If they use similar style to english I would think you could get by only knowing a few pronouns, articles, and verbs. If its something like chinese you want to know sentence structure, locations, numbers, pronouns, verbs, articles.

i have to agree with coheed, in languages that have the same basic structure as your first language minimal understanding of the language can still yeild ok conversations. i believe my chinese teacher says you should understand and know something like 5000 characters and words in chinese before attempting a conversation, personally i think round about 1000 words and you can have a basic conversation

It depends on the language you,re willing to learn. As for English, many studies have been carried out.In one of them (supposedly by E.W.Dolch)a list of basic sight vocabulary of 220 words (110 easier&110 harder)and a list of the first thousand words for children's reading have been recommended.One who is learning a foreign language, I think,should follow the rules of thumb of the children of the same language.

Its not quantity although it sure helps. Which words you know helps, but more importantly how they conjugate and have to be combined is just as important. Many languages have past tense, future tense, future participles etc. But some need to be conjugated for pronouns. French has different verbs for 'go' for I, you, you formal/collectively, he/she, we and they. That's 6 different "go"'s. Its not as crazy as it sounds. Even in English, we have : "I am", "You/They/We are", "He/She/IT is" that's just "to Be". We conjugate too. THEN you have to combine for tense. I took French for years but never knew how to say "By Friday, I will have been here a week". By the way, in French, gender is given to inanimate nouns. A man's tie is feminine. Une cravate. And that's all just French (most romance languages). Japanese conjugates in completely different ways. I once heard a radio announcer say to a Japanese translator, "Ok, translate this.... I will conquer.." And the Japanese translator couldn't translate. She had to know what he was going to conquer before she knew which conjugation of conquer to use. Whuh? As you can see its not just word-count that matters but the dynamics of how the words work together that makes the difference. Conversation is best for learning this. The progression should probably go in the order you listed them. Conversation with someone patient, reading comprehension, and lastly rapid fire comprehension of television.

It is not the number of words you know that make you fluent, but the ability to manipulate the language in order to explain something for which you do not know the word. Words are easy to learn: grammar patterns are the key to language acquisition. eg If I do not know the word for "machine gun" in italian (which is "mitagliatrice" by the way") but I can ask. "Cos'e' la parola per un fucile automatico?" (How do you say the word for an automatic gun?) I can quickly obtain the word I need. But knowing "mitagliatrice" is useless if I cannot express the thought I need to get across.