7990 is terrible for mining. Overpriced and major heat dissipation issues. Dont bother getting into to the game with anything less than the new r9 series, they are way cooler and a bit more efficient than the venerable 7950.
They could be had for a while at a great price working out cheaper than a 7950 per core. Undervolted and slightly underclocked they can produce 630kh/s per core at 70C - 75C for under 400W. No complaints here.