Robert Duffy, id Software programmer, updated his .plan about DOOM 3 settings and how they will work, discussing specifics about the "ultra" quality setting as well as differences in how certain data are handled differently by ATI and NVIDIA hardware. He also mentions that the game is capped at 60 FPS (except when running demos as a benchmark), discusses the software used during development, as well as a bit on a hard drive nightmare encountered along the way. Here's a bit:

Image fidelity is dependent on what quality level we load the textures at.

In Ultra quality, we load each texture; diffuse, specular, normal map at full resolution with no compression. In a typical DOOM 3 level, this can hover around a whopping 500MB of texture data. This will run on current hardware but obviously we cannot fit 500MB of texture data onto a 256MB card and the amount of texture data referenced in a give scene per frame ( 60 times a second ) can easily be 50MB+. This can cause some choppiness as a lot of memory bandwidth is being consumed. It does however look fantastic :-) and it is certainly playable on high end systems but due to the hitching that can occur we chose to require a 512MB Video card before setting this automatically.

High quality uses compression ( DXT1,3,5 ) for specular and diffuse and no compression for normal maps. This looks very very close to Ultra quality but the compression does cause some loss. This is the quality that for instance the PC Gamer review was played in.

Medium quality uses compression for specular, diffuse, and normal maps. This still looks really really good but compressing the normal maps can produce a few artifacts especially on hard angled or round edges. This level gets us comfortably onto 128MB video cards.

Low quality does everything medium quality does but it also downsizes textures over 512x512 and we downsize specular maps to 64x64 in this mode as well. This fits us onto a 64MB video card.