The official benchmark results of amd hd 2900 XT were posted in comparison to 8800 GTS 640 MB.
The Ati one scores better in comparison about 20% more on average.
Eg:3Dmark 06 ----Ati - 11500 --Nvidia --- 9800
But the real performance crown should be between HD 2900XTX and 8800 GTX or whichever card Nvidia is going to launch - like 8800 Ultra or any.
H/W specs wise Ati's is way better which should reflect in higher resolutions but i am not sure.
And did I mention the cards support HDMI audio and video too.
B/W Ati's mid range cards better offer more performance unlike the disappionting Nvidia's mid range offer.
Source:
http://www.dailytech.com/article.aspx?newsid=7043