What is the fastest single card?
dont forget 7990 uses alot of power and will get very hot.
I've got R9 280x it's get 800Kh/s I got 3
And i think it's the best choice.settings and OS I can only get my XFX r9 280Xs to pull 705mhs each and my Saphire 280Xs to pull between 550 and 650 each have 5 XFXs and 3 Saphires
windwows 7 32 bit and cgminer 3.7.2
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer.exe --scrypt -o stratum+tcp://mona.2chpool.com:5555 -u xxxxx.x -p x --thread-concurrency 8192 -g 2 -I 13 -w 256 --auto-fan --temp-cutoff 90 --temp-overheat 85 --temp-target 75 --gpu-memclock 1500 --gpu-engine 1027 --gpu-powertune -20 --gpu-vddc 1.000
http://imgur.com/LHTLKmt