There shouldn't be a big difference. Factory overclocked cards like the two you mentioned usually have their speeds increased somehwere around 5% and should perform about that much better than the standard version out of the box. So if the price is more than 5% higher (which it usually is), finding a good deal on the standard version delivers better value.
Either card (or the basic 6950) should perform pretty well in Skyrim at 1080 and lower resolutions. He may not be able to turn every setting to max, but it will do very well. Cheers!