among other things the 2900xt has more of than the 8800 (memory interface and stream processors) it also draws more power than the 8800ultra!
http://www.tweaktown.com/articles/1131/14/page_14_power_consumption_tests/index.html
http://www.tweaktown.com/articles/1131/14/page_14_power_consumption_tests/index.html