I have the same problem with a very similar card.

Card is Gigabyte 7970 Ghz edition.

In addition to what Benjie did, I also flashed the 7970 OC version, switched to Linux, changed memory, removed one GPU card, switched to all Catalysty 12.8, 13.1, 13.4, 13.5 (beta) , 13.6 (beta), and SDK 2.7 and SDK 2.8, cgminer 3.11.2 till 3.2.2.

Still I can't get more than 540 kh/s.