They use a stock x6800 in that review....which draws a max of 75 watts at full load.
So if the entire system draws 492...assuming the PSU is 85% efficient at this amount of load...
GFX = 492 - CPU - psu efficiency loss - mobo - other shizzle such as drives/fans
GFX = 492 - 75 - 74 - 40 - 40
GFX = 258watts
That's a very rough approximation but it sounds about right about in the ballpark, it's most likely 20 or so watts lower than that.