If you currently have a Xbox 360 with under 8GBs of memory then you might want to look at purchasing a new hard drive or upgrading as Microsoft have just revealed on Xbox.com, that the games multiplayer will require at least 8GB to play:
“At a minimum, you’ll need at least an 8GB USB Flash Drive or an Xbox 360 Hard Drive to play Multiplayer mode in Halo 4. For an optimal experience playing with an Xbox 360 Hard Drive is highly recommended. For Xbox 360 4GB, Arcade and Core consoles, the Xbox 360 Hard Drive is sold separately.”
Most of this will be taken up by the games maps and content, for which 10 different maps will be available at launch.