I just recently purchased Hearthfire and only now have I had the opportunity you actually do it. I have only one question. Can you choose the land where to build your house? Or only where you receive the letter from the Jarl?

You don't need the letter, but you only have one plot of land available in each hold - near Pinewatch in Falkreath Hold, near Loreius Farm and a giant's camp in the Pale and between High Gate Ruins and Ustengrav in Hjaalmarch.

So I can build more than one house?

Yes you can get 3 plots and build three Hearthfire homes...

Sweet!