Activision’s Community Manager has confirmed that the 3GB install for Ghosts will be only for the campaign mode. If you plan on playing multiplayer, then you will not need to install this at all.
Ghosts will require a little space and time to install its single-player campaign data. This should be less than 3GB of space, but you’ll want to make sure you’ve either cleared space or you’ve got an appropriate USB flash drive ready.
Ghosts will play on an Xbox 360 with an internal 4GB flash drive – just use that built-in storage space for the installation.
Amirch also confirmed that the Free Fall bonus map will not take up much space – it’s only 250MB download.