So, 2 MB for the display. And then there are the textures for the walls, plants, people, monsters, etc. And the vertex data for each of the models on the level. And the level itself. And then there are the light maps for static lighting. That's at a minimum. After that there is the memory required for the AI. Much of the models and textures won't be used in any given scene, but might get used in the next frame, or the next half second, and to avoid long pauses when a new model is loaded they all have to be in memory. If Half Life could have been done in less memory, I'm pretty sure they would have done it. It probably started off using a lot more memory and they probably spent a lot of time getting the memory down so that your mid-class machine could play it.