I still have a little over 12 GB left on my 20 GB hard drive for the XBOX 360. I don't think we have to worry too much about memory, I mean with all the update info and arcade games plus multiple saves for around 15 full games, and I haven't reached half of the hard drive
!!!
But if it becomes a problem, can't you save your old save games into an USB-storage device or a computer external Hard drive??
Besides that, the save files for Fable TLC weren't that big either ... I guess it is going to depend on how much DL content will actually be and how much space it takes up.