For a multimedia box, definitely go for the quad. While you may not see a huge advantage one way or another with duo vs quad with some basic local use, the quad is going to allow you to scale more. For example, with Media Center, you can add extenders to be able to watch content off the main machine on another TV elsewhere in the house. Windows spawns a modified remote desktop process to support extenders, so with a quad, the process could take a full core, or even two without impacting the local machines ability to record and watch a show.