7990s. you wont' like the temperature. I constantly run above 90 c. can be tricky to keep it running. will burn a lot of boards.. gl
You need to undervolt the core with MSI afterburner beta. My setting with 1.0V, 950/1000MHz, it hashes at 1320kH/s. The temperature varies depending on the thermal paste of the cards.
I'm curious, what temps do you get?
no sense to buy a 7990
280x = 750 kh/s - 275w
7990 = 1300 kh/s - 500w
7990 = about 70% faster - about 80% consumption = fail
That's not that bad. It saves you a PCIe slot too.
It only becomes terrible if you factor in the GPU costs.