Last January Tom's hardware published an article about actual power consumption while playing games. A typical user with a pc having just one video card rarely used more than 300 watts at the wall outlet (mains). There are some exceptions, especially with very serious overclocking, high end video cards, high resolutions, and high settings.
The ATI Radeon HD 4770 is not a high end video card. Instead, it is an excellent general purpose card that is energy efficient. It just happened to perform reasonably well when two of the cards were used in Crossfire mode.
Here is a link to Tom's Hardware original review of the 4770:http://www.tomshardware.com/reviews/radeon-hd-4770,2281.html
Here is a link to Tom's Hardware review of the 4770 in crossfire mode:http://www.tomshardware.com/reviews/radeon-4770-crossfire,2288.html
NOTE - There are quite a few charts with results for a variety of games.