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.
This keeps being spewed around as if miners are just blindly buying ineffective hardware.
The fact is that the 6990 is a great miner because:
1. It has two GPU's in 1 card
2. If your motherboard has 2 PCI-e slots, you can turn them into 4 gpu's worth of hashing power.
3. If your motherboard has 4 PCI-e slots, you can turn them into 8 gpu's worth of hashing power.
In the case of single core GPU's, you would now have to buy 2 computers with 2 power supplies, processors, motherboards, RAM, casing etc.
So in the end you save space & don't have to buy a second rig to house more cards & you get greater resale value.