I bought an XFX 7970 DD BE at launch and max stable clock was 1200/1710 on 1.21 Vcore, stock Vram.
I've heard that recent versions of the card are voltage locked - is yours? How are your temps? Do you have a good enough Power Supply?
Use MSI Afterburner to handle your overclocking, by the way.
And as always, overclocking depends on luck of the draw - you may have just gotten unlucky and not gotten a card that OC's well.