depends onthe card. Obviosly binning would be important however the are a large amount of cards doing 1ghz on 1.2v
http://www.xtremesystems.org/forums/showthread.php?t=235693
HD5870 average OC: 1027 / 1286 @ 1.27v
The concept of average is anathema to XS. You're talking about the most hardcore overclockers in the world, guys to whom 24x7 stability isn't even a concern. Any results gathered from such a sample should be treated as the upper bound, not the average.