For x2 6990's you need a 4ghz+ processor or else it will get severely bottlenecked.I've seen people hit 4ghz with the 980x but you will need a good cooler to get there.And I think you will need more than 4ghz to not get bottlenecked.
I have the same cpu and I have 3 GTX 580's in 3-way SLI and no problem , that cpu has an unlocked mulitplier and can easily be overclocked especially if you have a good overclocking friendly MB.