We sat down to think through the project. Here's the list of functional requirements we came up with:
Most important, this system must have all of the resources necessary to be a good gaming platform. It must be fast enough to handle any current game at least reasonably well, have enough ports (and conveniently located) to handle any combination of game controllers, and so on. It must have audio and video support suitable for any game we decide to play on it.
This system must have all of the resources necessary for use as a general purpose system. No one who builds this system for gaming should have to own a second system for other purposes. That means, for example, that the system must have a FireWire port for downloading DV video from a camcorder (and sufficient disk space to edit that video). It must have a display and other external peripherals that are suitable for general use, as opposed to only peripherals optimized for gaming. This system must be capable of handling all but the most specialized tasks with aplomb. Two plombs, even.
Although portability is a secondary issue, it remains important for the gaming PC to be easily portable. Size and weight are both considerations. We set our upper size limit at a standard mini-tower case, which is small enough to be easily portable but large enough to contain several drives and whatever other components we might wish to add later and with enough volume to eliminate any problems with ventilation and cooling. We set our upper weight limit at 30 pounds, give or take, for the system unit, which is light enough for Barbara to pick up with one hand.