"* Hard Drive: 18GB free hard disk space"?
I call *bull shit* - although, I know, I've seen this with my own eyes on the GFWL site...
But it MUST be a typo.
Think of this: San Andreas unpacked to like 4.68 GB. It carried with it many upon many more vehicles types, weapons types, types of trees, and less efficient than current compression methods textures that all of those mentioned found in IV, even though they were half the resolution (and less).
Liberty City 08 is about 4/5 the size of San Andreas.
Double the texture resolution (biggest part of any art assets) to accommodate the new gen, take out some vehicles, take out some trees, take out some overhead by better compression techniques, and add some back in by adding a few new stations and making every one slightly longer and suddenly San Andreas is 8 gigabytes.
Disk Space Required for GTAIV? IMO - 8 Gigabytes.
The ONLY thing that would justify an 18GB space recommendation from a game that comes on ONE Dual Layer DVD is that the game truly IS built to last (as rockst5ar said - IV built to Last) - and that the future games will CONNECT to this current game in full-priced expansions. (maybe not only thing, but way more likely than a complete texture overhaul)
Yes, I am saying that the only thing that makes me believe 18GB is that vice city and san andreas from this gen will require IV to be run - they will be expansions. Makes sense when you consider #1: Liberty's airport is now on the far east side of the city; since VC is south and SA is west, this makes every flight long and more realistic.
There are probably a lot of other things that could support this, but I'll let the discussion grow and see what you guys think. The moment I saw the new position of the airport in liberty, I knew "built to last" meant something more than just DLC scripts on 360.
Unrelated: Bohan is being expanded as we speak.