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The 5xxx series doesn't scale all that well in crossfire at all. You may well be better off selling and getting a new card.
I'd highly recommend the 6950 for price : performance, and also (as mentioned) the fact it can be flashed to a 6970 bios.
Little bit of an exaggeration there. The 6900s scale better, but the 5800 scaling is by no means bad.