It depends on the resolution you are using as well, you start to see the impact of the lower bandwidth of 2x8 vs 2x16 at 1600x1200, with significant impact starting at 1900x1200 and higher. Do a google search on 4850 Crossfire reviews, there is one done by a site that tested for this with a P45 board vs an X48 board.
See someone beat me to it. The article shows it well.