An upcoming Xbox 360 title due out in 2006, Gears of War is being built off the Unreal III engine. Looks beautiful and has some potential. Until we can truly see the Xbox 360 in action though, everything about that system is potential. And I don't mean an E3 demo, I mean a finally, fully developed product with a completed launch title.
Check out Gears of War though, especially the trailer. This might be the first title to actually give need to going out and buying the next Xbox when it's released should it's style and gameplay match the appeal of its visuals.