There was a great article here a few months ago on this very subject, detailing getting Halo 2 to run in true HD, properly using the extra RAM of the devkit, an

There was a great article here a few months ago on this very subject, detailing getting Halo 2 to run in true HD, properly using the extra RAM of the devkit, and the engine hacks required to do so (since it basically statically allocated a handful of regions and couldn't dynamically go "oh look more RAM!"). It was a good read!