The best of the AMD lineup and from the manufacturer with the best warranty and reputation on the planet, def in for one, two would be even more kick butt.
Well, firstly, XFX is awesome for standing by its product with a double-lifetime warranty. That has always been something very impressive to me.
Secondly, the 6990 is an impressive product on its own right. Being a dual-GPU card, it carries a lot of the beef all on its own. It truly is the fastest card on the market.
Regrettably, the last XFX vid card I owned was a 6800gt AGP, but the card lived a long and healthy life. A 6990 is certainly a good way to get reacquainted