Here are screenshot comparisons of Gun on both systems, using a high definition TV. Note that of course if your COMPUTER MONITOR is low resolution, you won't see the detail difference. The screenshots I show can only be as good as your monitor is.
I took these pictures with a digital camera. Note that my hands aren't perfectly steady, so there is a little jitter here :)
Regular XBox version of Gun, cut screen
XBox 360 version of Gun, cut screen
These are cut scenes, and the cut scenes were done "widescreen" in the original XBox so it makes sense that these would match up in that sense. Note that these are "free running video" segments so it was hard to take the photo at the exact same time in both.
What's interesting in this set is that usually the XBox 360 is lighter, but I happened to catch this pair in different spots so the XBox 360 version is darker. Look, though, at the great detail in the XBox 360. The necklace, the boards on the left, even the struts in the umbrella.
XBox 360 Comparison Screenshot Set 1
XBox 360 Comparison Screenshot Set 2
XBox 360 Comparison Screenshot Set 3
XBox 360 Comparison Screenshot Set 5