Sorry, as your board is an "SLi" compatible board, 2 x1900xtx's or any ati crossfire combo will not work. SLi is NVidia's system of linking 2 NVidia cards and is not compatible with ati. As Sirheck said, one ati card will work fine but you just wouldn't be able to use the interface to link 2 cards.
On the other hand 2 x NVidia cards will work sweet. If you seriously need 2 x x1900xtx's in crossfire then you will need a new motherboard.