Thank you VERY much Mr.Borek!
That's what I needed! Unfortunately There was a misundersting and I set on edge you. I apologize.
Now, it's time for me to do my part. I'm going to explain everyone the proceedure, so that every fellow who might get in touch with such a particular problem: that's, in my opinion, what we ALL have to do here.
Here we are:
We have two different kind of reactions. The former is a neutralisation
H3PO4 + HPO4-- = 2H2PO4- => [H2PO4-]=0.4M and
The latter is a simple buffer solution
[H+] = Ka2 * [H2PO4-]/[HPO4--]
5.012E-8 = 6.32E-8 * 0.4/x => [HPO4--]'' = 0.504M
So, in the end, the total amount of HPO4-- required is
[HPO4--] = [HPO4--]' + [HPO4--]'' = 0.2 + 0.504 = 0.704M
Well, it is easy, isn't?