Running 6 x Asus R9 280x with two more arriving any day now.
745khs out of the box on stock. Voltage is unlocked so you Undervolt to save power and heat which lowers fan noise as well.
Don't recommend the Sapphire Radeon R9 280X Dual-X as they throttle at 72c or something silly.
Go with the ASUS, you wont regret it. I haven't
I'm running Gigabyte and Sapphire 280x @ 730 and 750 Kh/s.
Sapphire doing the right job.
http://oi40.tinypic.com/ws7plx.jpgI think you have plugged both the cards in the motherboard...
maybe you should use riser cables and keeps the cards a little apart to maintain the temperature issues, then you'll have good performance on both
