We've seen GTX 580 working at 895MHz, GTX 570 at 880MHz as well as viewed results of Radeon HD 6970 clocked at 980MHz, and 6950 at 1010MHz
If these figures are common overclocks. These would be the gains.

GTX580 772 -> 895 (15.9%)
GTX570 732 -> 880 (20%)

6970 880 -> 980 (11%)
6950 800 -> 1010 (26%)