Question: We've got a dispute on the point whether the phrase should be "What you are talking about." or "What are you talking about." in the affirmative sentence.

I was arguing that it should be the former, and the word-order is not so important in that phrase, is it?

Help us solve the dispute, please :)

Answer: 
In English, "What you are talking about" is not correct at all unless you were saying:

> 
> "What you are talking about is....."
> 
> 
> 

It would always be "What *are* you talking about?" in a question.
In English, an interrogative clause is marked by **subject-auxiliary inversion (SAI)**, a process in which a subject and auxiliary verb switch places. Here, the auxiliary verb is *are*, and the subject is *you*:

1. Declarative clause (subject and auxiliary in basic position):

> 
> **You are** talking about *something*.
> 
> 
>
2. Interrogative clause (subject and auxiliary inverted):

> 
> **Are you** talking about *something*?
> 
> 
>

A typical question takes the form of an interrogative clause. When it contains an interrogative phrase (containing or consisting of a *wh*-word such as *what*) that is not in subject position, that interrogative phrase is typically **fronted**, meaning it's moved to the front of the clause:

> 
>  1. **You are** talking about *something*.  
> 
>   2. **You are** talking about *what*?  
> 
>   3. *what**i* **you are** talking about \_\_\_\_*i*  
> 
>   4. *What**i* **are you** talking about \_\_\_\_*i* ?
> 
> 
> 

In 2, *what* is the object of the preposition *about*; when we move it to the front of the clause, it leaves behind a **gap** which cannot be filled. In 3 and 4, the *wh*-word is understood as if it were the object of the preposition *about*, even though it doesn't actually occupy that position. (I've marked both the gap and the *wh*-word with the index *i* to indicate that they refer to the same thing.) 

In 3, we have a *wh*-relative clause. It could be used as part of a larger sentence, but it's not a complete sentence itself, so I didn't capitalize it or put punctuation at the end. In 4, because the subject and auxiliary have switched places, we have an interrogative clause--a complete sentence and a question. 

Unlike an interrogative clause, a *wh*-relative clause cannot have the subject and auxiliary inverted:

> 
>  5. (I have) no idea [ *what**i* **you are** talking about \_\_\_\_*i* ]*wh*-relative .  
> 
>   6. \*(I have) no idea [ *what**i* **are you** talking about \_\_\_\_*i* ]*wh*-relative .
> 
> 
> 

For this reason, example 5 is grammatical, while example 6 is not. (I've placed the words *I have* in parentheses because they were omitted from your original sentence, but it makes no difference grammatically.)