G4 gave Halo 3 a perfect 5/5 score. Unlike most sites, they considered multiplayer first and campaign second, but they say either way, the game is pretty:

Ever since the multiplayer Beta, Halo has come under fire for its graphics, and comparisons to Halo 2 and Gears of War have been numerous. With the release of Halo 3, many of those dissenting voices will be silenced. Halo 3 is beautiful. And much of that beauty lies in its depth.

How much game is in Halo 3? G4 calls it Bungie's 'Orange Box'. That ought to answer that.