One DVD isn 't going to cut it soon. The XBox version of RAGE is going to be 2 DVDs, and going to have "rubbish" graphics compared to the PS3 BD version.
I heard that was an over-dramatic assertion. Carmack has clarified that it will look "rubbish" in areas that aren't frequently traffic'ed and where they can get away with a smaller texture (such as caverns, etc)
All of the other parts of the game will look comparable to the PS3/PC