Just a heads up, you can't play the campaign online with another player with just the 4GB of memory that comes with the Xbox.
I had the 4GB without a hard drive and tried to play co-op campaign over Xbox Live, and was unable to do so. Using a USB won't work either, as I was using that in place of a hard drive.
You need to have a physical hard drive inside your 360.
Fun facts go here.