no sense to buy a 7990
280x = 750 kh/s - 275w
7990 = 1300 kh/s - 500w
7990 = about 70% faster - about 80% consumption = fail
For me
7970 = 750 kh/s - 260w = 2.88 kH/s/W core/memory: 1.113/1.5V, 1065/1500MHz
7990 = 1300 kh/s - 400w = 3.25 kH/s/W core/memory: 1.0V, 950/1000MHz
So 7990 is better. We can use 4x7990 but only 7x7970 on a MB.
If we can undervolt the memory, it is even better.
The temperature I get is from 65C to 92C depending on the position of the card and quality of the GPU and how the thermal paste was applied.
Some times I have to undervolt to 0.96V so that it runs at 860/900MHz to keep temperature below 85C. I am thinking of reapplying the thermal paste, but not sure if it will void the warranty.