I didn't know that Dead Space was originally destined for the XBox! It's a great game, and I don't doubt that the XBox could have run it fine, though of course with lesser graphics than the final (360) version.
Turning Point: Fall of Liberty, and Perfect Dark Zero, are two other games that were originally announced for the XBox, but were kept back instead to appear on the 360. Unlike Dead Space, however, these games were definately not worth the wait