The only 5 series card that get over ~400Mhash/s are the dual gpu cards ie. 5970.
I wouldn't waste money on the gpu you selected. I'd buy a 5830 for less than $100 on ebay and get 300Mhash/s per card.
That's the thing. My current 6970 only get's a little over 300Mhash/s. Why does a later generation also produce similar results?
My 6950 unlocked to a 6970 gets 400Mhash/s. Make sure in CCC that under performance "AMD overdrive" power control settings should be at +20%.