The observing ones among you immediately notice that there will be no XTX Crossfire card, this is correct. If you want to go Crossfire on the 1900 then that XT is the only solution you'll have. Mixing XTX and the Crossfire XT would not make sense as the XTX would drop down in speed to balance the XT performance. So if you want to go for Crossfire now or in the future then just stick with the X1900 XT.
http://www.guru3d.com/article/Videocards/315/3/
http://www.guru3d.com/article/Videocards/315/3/