If your handy enough you can try flashing the bios to a gigabyte brand or other. Some people have reported successful increases in hashrate after doing this. Mostly XFX and mostly related to litecoins.

Ive seen threads where people are stuck at the 500Kh/s mark and after flashing they apparantly hit 700Kh/s+

EDIT: Full disclosure I have 3 XFX 7970's and I must admitt, they arent the greatest! I havent flashed them yet though! Bought as part of the same consignment, 1 out of the 3 cards was artifacting after installing, I immediately returned it to the merchant. Still awaiting word back about the replacement. They are quick as hell to respond when they are taking your money, when your looking for it back or something similar, it takes them an age to even email you.

I think i'll stick with the better brands from now on, XFX is most definitely on my shit-list