I'd be interested in knowing this as well. Is it also a real thermal consideration to have these OC'd cards?
Differences will be marginal, so if the price difference is marginal too, it is worth it, if there is a clear difference in price and a hardly noticeable performance gain, then no. In this short list of 670 cards, the Palit and Gainward cards @1.006 GHz look attractive.
These last two choices are hardly more expensive than some other cards @ 915 MHz.
The performance gain will be best noticeable when you do a lot of MPEG2-DVD encoding from HD material (scaling), or do FPS adjustments (frame blending) on export, since both are accelerated and when you use Maximum Render Quality.