The screenshots of the game are not
how the game will look, as they take screenshots of games at much higher resolutions than the Xbox can pump out.
But here's some candy for you: Image1 Image2 Image3 Image4 Image5 Image6 Image7 Image8
And you can get some videos from Here.
A clip of Halo2 Multiplayer: http://media.xbox.ign.com/media/482/482228/vids_1.html?fromint=1
And here's a large video of "The Making of Halo2" showing the wonderful funderfull world of game making: http://download.microsoft.com/download/7/a/a/7aa5f21b-09d6-41b6-a647-b3f2da348db
And here's some voice acting outtakes: http://www.bungie.net/Games/Halo2/page.aspx?section=AudioVideo&subsection=VoiceO
Two more images:
And here are some high resolution scans of the Halo2 preview in Junes OXM magazine: page 1 page 2 page 3 page 4 page 5 page 6 page 7
And one last image: http://img14.imageshack.us/img14/8044/Halo_Resize.jpg
And all the current screenshots can be found on Bungies site in 1600x1200: http://www.bungie.net/Games/Halo2/Gallery.aspx?Gallery=Screenshots&page=3