Just posted by Sketch:
"...please be aware that installing Halo 3 to your HDD will actually result in
longer load times and thus a less optimal experience than if you run the game from the disc drive. This happens due to the way Halo 3 already uses a HDD to cache data and once you do an install, you are copying between two different areas of the same drive as you are loading a map.
If you just plan to play campaign or offline content, you may feel that the tradeoff is worth it in order to have a slightly quieter Xbox 360 and reduced impact to your disc drive. However, if you play Halo 3 online, please be aware that
increased map loading times can have a negative impact to everyone's experience. In some cases, it could take considerably longer to load multiplayer maps, meaning that everyone you are matched with is also stuck waiting for your maps to load. If people start to lose patience, you then have scenarios of players leaving parties and aborting matching because they don't want to wait. This adds to the problem, creating a cycle where more and more people have a harder time getting into the games they want to play. To avoid this negative situation, we are advising that Halo 3 players do not install the game to their HDD."
Read on for greater details.