I checked a few references. A single Radeon HD 4870 graphics card will use about 130 to 150 watts and 12 or 13 amps at full load. Different tests produce slightly different results. The variations are normal.
A typical system would probably use about 300 watts at full load. Here's a link to a technical review with a power consumption charts for idle and full load for a variety of video cards:http://www.anandtech.com/video/showdoc.aspx?i=3341&p=22
Note - The wattage in the chart is the combined total for an entire system.