By far, ATI video cards are more power hungry than nVidia. On the premium end the X1900XT is a slightly more powerful card than the 7900GT, but it consumes more than twice the power; 108w vs. 48w.
On the entry level side of things the X1300 uses about 31w, not bad. But the 7300GS uses only 16w of power. Of course that doesn't take into account performance. I actually don't know which entry level card performs better, nor do I care since entry level cards are not on my shopping list.