This is a screenshot comparison of Halo: Reach(pre-Alpha & pre-Beta) and Halo 3, we'll be discussing the improvements in terms of graphics Bungie has managed to achieve in Halo: Reach over Halo 3. Some of the screenshots for Halo: Reach are from Alpha or Beta stages of the development but Reach already looks significantly better than Halo 3. Bungie have themselves admitted that they have to up the ante in terms of graphical prowess for their upcoming and possibly final game in the Halo universe.
The Assault Rifle looks significantly pale in Halo 3 when compared to the one in Reach and with better textures, improved bump-mapping, finer edges and color grading it stacks up against one of the best looking weapons.
Maybe a closer look at the spartan in Halo: Reach would have made the graphical improvements more apparent but look closely at the shiny metal surface behind the spartan where you can see some great specular reflection.
Both look well anti-aliased but the spartan in Halo: Reach has a higher polygon count and you can also notice the specular highlight on the helmet. Also the textures on the armor look way better in Reach than in Halo 3.
We'll keep updating this post building up to the beta release.