The Palazzo Medici is a Renaissance palace located in Florence, Italy. It's famous. It's also one of Ezio's playgrounds in Assassin's Creed 2, provided you pre-order either the PS3 or Xbox 360 version of the game.
The downloadable Palazzo Medici area is a GameStop pre-order item. According to Ubisoft, the Medici family palace contains "secret treasure," something for Ezio to get his bloodstained hands on. The offer is good for both online and in-store purchases.
It's worth noting that Palazzo Medici is one of the three bonus quests included in the game's Black Edition, but that one's a whopping US 115. If you really wanna play through it then this is the better choice, unless you have the extra money to burn.