That's because they're dual GPU cards. So crossfire / SLI on one board. e.g the 6990 is 2 6970 GPU's.
You'd be pushing it with your PSU plus those cards are horribly overpriced and very noisy. Also they are often downclocked versions of the original GPU. They're not really desirable cards at all unless space is an issue.
You'd be pushing it with your PSU plus those cards are horribly overpriced and very noisy. Also they are often downclocked versions of the original GPU. They're not really desirable cards at all unless space is an issue.