A standard personal computer can be equipped to run simple virtual environments. The processing speed of the computer will determine the maximum complexity of the virtual environment you can build. In order to maintain a decent illusion of reality, the spatial aspects of the sensory displays must be recalculated and updated more than 20 times each second. In order to figure out how much complexity can be included in your virtual environment, you need to be able to describe the speed of your computer in VR terms.
Most of 3D graphics is based on building objects out of triangles or other simple polygons. For computing visuals, a convenient metric is the number of polygons your computer system can draw in one second. Since the computer must draw separate views for 2 eyes at least 20 times each second, you must divide the number of polygons per second by 40 to determine the maximum number of polygons that may be simultaneously visible in your virtual world. Thus, a computer that can draw 50,000 polygons per second will be able to support a virtual environment containing a maximum of:
50,000 polygons per second
------------------------------ = 1,250 polygons
2 eyes * 20 views per second
Since 1,250 polygons is not very many from which to build a whole world, and since the polygon drawing speed quoted by hardware and software manufacturers is often optimistic, you generally need to either design very simple worlds, or get extra graphics hardware to help out your computer.
In more complex VR systems there may be four or more computers running one Virtual Environment. There would be a computer for the basic program commands, a computer to run the graphics and sound, a computer to detect collisions within the Virtual Environment, a computer to follow the tracking of the individual in the Virtual Environment, a computer to operate the various input devices for the Virtual Environment, and additional computers to process other Virtual Environment factors.