For those of you who are interested in the technology that goes into making a cutting edge engine like id software's Doom 3 engine, Intel has a bit of a good read in store for you. Make some time and get some coffee, there's plenty of info to pour over.
"The articles below provide an overview of the SIMD optimized rendering pipeline of animated models which is equivalent, but superior in performance to the one used in the computer game DOOM III. This improved rendering pipeline is implemented in the computer game Quake 4. Most of the rendering pipeline runs on the CPU while several of the steps in the pipeline could run on a GPU as available on many of today's graphics cards. However, running most of the pipeline on the CPU improves compatibility across a wide range of systems."