Highly reccomended to get 2 of the same series, just like with Sli, otherwise performance will drop or it may not be compatable at all with some newer models and some of the older "first gen" crossfires.
ya if he wants to Crossfire it should be two of the same cards, but if you wanna save some cash, why not buy an hd2900pro and then Flash it into an hd2900xt