ATI's watt usage for current video cards
http://www.geocities.com/rhelic22/ATI-Watts.gif

Two things becomes clear here, faster cards need more power and improved designs are more efficient with that power.  So it seems when a design is created, you can pump large globs of power at that design to get more power, but if you simply re-architecture the card with newly discovered technology, you can be more efficient.  When you look at the Raedon 9600 it's very efficient but in order to get more speed (with more power) you have to quadrouple to barely get double the speed, EXTREMELY inefficient.  But with a totally new design (the x800) you get way more speed without wasting as much power.

Now you see why video card companies are re-designing their video cards every 8 months.  Because if they didn't, our video cards would be eating up 500 watts by now.

PS: I didn't do the benchmarking, I stole the data from Anandtech's article and compiled in into 1 graph.  You can read the original article at http://www.anandtech.com/news/shownews.aspx?i=22655

UPDATE: it seems I can't include Geocitie images as they use some type of cookie or referall url technology to prevent linking.  Please click the link above.