The resolution of the monitor you play on. An old square monitor is 1280x1024, where as a 24 inch LCD may be 1920x1200, or a 30 inch LCD may be 2560x1600.
The higher the resolution the better the GPU you may potentially need.
If you are on the monitor you will be using now, just right click the desktop and go to the display properties and it will tell you what resolution you use.