According to that site, there are 31 xbox games that support HD (720p and above, 480p is ED). Of those I only own Xmen Legends, and haven't gone back to play it since I got my HDTV. I'll let you know if it's any better (how much better can such low-poly stuff be, really?).
By default, unless you have a good chip in your TV the 720p/1080i games will look better than the glut of 480p games mentioned above (Ninja Gaiden, SC:CT, etc) as they will be more sharply defined and not subject to 3:2 pulldown and/or stretch modes.