I think the 7970 comes very close in performance to the 6990. See here http://www.youtube.com/watch?v=iRT9rOOsb78
Right, but the 6990 is a dual GPU, while the 7970 is a single GPU. The rumored 7990 would be the next dual GPU board, and be the successor the 6990.
The 7990 would be the equivalent to the 6990 in the 7000 series.
Each board being faster than the last should be a given.