The 6990 is a dual GPU card. It's literally just two 6970s on one board. The equivalent dual GPU card for the 7000 series won't be released until later this year. The 7970, which will be combined for the 7990, is available, however, and you can just get a pair of those.
Crossfiring two cards will generally produce better results than a dual GPU card because heat is much less of an issue and the cards can be clocked much more aggressively.