You couldn't have seen the 6990x2, since it doesn't exist. 6990's are NOT good for mining in terms of euro/MHash or euro/Watt, especially for you, OP. In your situation, I would go for a 5830, you could crossfire it with your 5850 for gaming, and it would be the best value for money you can get. Where are you located, I might be able to help you with prices. I just got 3 5830's in Holland, but there's even better prices in France, Germany/Austria, etc.