Dont ask how i found it, but its from a trustworthy source.
It just about confirms that the Downloadable content for the Xbox 360 will be another city 0_o 
Maybe San Andreas, maybe Vice City, maybe even London
Heres the link
http://www.playthree.net/2008/04/cvg-gta-iv-xbox-dlc-will-be-bigger-than.html