The graphics will be fine. Halo 3 was using a relatively old engine (upgraded though). But Bungie has not received the graphics praise they actually deserve for Halo 3. Sure, it lacks AA and runs at Sub-HD, but the amount of special effects, transparencies, reflection, lighting, and especially the real-time rendered ships of a mile long or bigger and amazing draw distances in the landscape, they are actually features of top notch optimization.. And that's not even taking the AI, amount of enemies and physics into account. And they optimized the DVD as well and that's why installing the game ruins the loading times and performance. This is supposed to be a new engine from scratch. Also, see this comparison between the three halo games, and you'll see how much the graphics have actually improved. I have no doubt that Halo Reach will be top notch on gameplay, but graphics will also have a leap above Halo 3.
Truth does not fear investigation
