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.
You'll note that both look very nice, with good detail on the weathered building, for example. But look at the "preacher's" face - the guy on the left. In the XBox version it is very plainly lit - bright spots on the face. In the XBox 360 it's much more subtle, much more realistic. I didn't use a flash on either picture, both are just the way they look.
XBox 360 Comparison Screenshot Set 1
XBox 360 Comparison Screenshot Set 2
XBox 360 Comparison Screenshot Set 4
XBox 360 Comparison Screenshot Set 5