I don't know... I don't really think that Desmond had known before he turned they key... I think that some effect of turning the key is why he remembered the past the way he did and know bit of the future now... I noticed how if he was in London he remembered bits from on the island... Now on the island he remembers bit from on the island that have not currently happend yet... Something happened when he turned the key is why he knows what is going to happen in the future now....

__________________
Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.