What does it mean?

Generally,being "at sixes and sevens"with somebody means that you are confused by them or you can't make up your mind about someone.

There is a mistake in your transliteration of the verse:

All you will see is a girl you once knew

Although she's dressed up to the nines

At sixes and sevens with you

'dressed to the nines" means wearing elegant or expensive clothes

"at sixes and sevens with you" -- appointments or dates which are scheduled for 6 or 7 o'clock in the afternoon --- or early evening, say, before a formal dinner (such as a meeting, a tea, a snack, a wine and cheese, a romantic little tryst, etc. etc.)

Though Crowbird is correct in his rendition of the verse, he is mistaken about its interpretation.

"Dressed up to the nines" does mean wearing extremely striking or elegant clothes. However the expression "at sixes and sevens" is nothing to do with the time of a date or a romantic meeting. It means to be out of tune, in disaccord , in contrast or different.

The meaning of the sentence is:

"All you will see is a girl expensively dressed but at odds with you" ("you" being the poor people of Argentina, and not someone with whom she had an assignation).

Eva Peron (Evita), the wife of the Argentinian president, was loved by the populace because she was young, beautiful and kind, despite the fact that she spent a great deal of money on her appearance and led an extravagant existence whilst very many people in the country were extremely poor. She died of cancer at 33. The verse conveys this discrepancy between herself and the majority of the people. The general message of the song is that she found no pleasure in an affluent life whilst trying to widen her horizons, and down at heart she was an ordinary girl who loved them.

http://homepage.ntlworld.com/alan.stuart/music/mad...

http://www.phrases.org.uk/meanings/sixes-and-seven...

it is used to describe a state of confusion or disarray

I always thought it meant at odds with you!

you dont know what to do