Here are the games that I know of that really show off the xbox graphics. A rule of thumb is to compare it to the ps2 version, if one exists. If the xbox version looks considerably better, then it probably takes advantage of the superior hardware:
Anyway, here they are:
Max Payne 1 and 2 (2 looks much better)
Rallisport Challenge 1 and 2 (ditto)
Panzer Dragoon Orta
Halo
Dead or Alive 3 and XBV
Morrowind (get Game of the Year version - not better graphically, but there's more content and little tweaks to the gameplay)
Splinter Cell and Pandora Tomorrow (Pandora looks better)
Ninja Gaiden (not cheap, but looks really nice)
Grand Theft Auto Double Pack
Wreckless
Chronicles of Riddick
Star Wars the Clone Wars
JSRF (possibly the best cell shading I've seen to date)
Project Gotham Racing 2
Crimson Skies (especially fun on Live - and it's free with the starter kit)
Top Spin (possibly the first fully polygonal large crowd)
State of Emergency (the ps2's processor was stretched to the max on this one; the xbox delivers a superior looking version without breaking a sweat.)
These are the only ones I can think of at the moment. There are probably many others. There are also many great looking games, but I did not include these because the ps2 ports were not significantly worse.