Only in high end graphic usage will you see an increase by using the dual 16x board. It's not even by that much. Before purchasing my DFI, I looked into the A8N32-Sli. Performance wise the DFI was at the top, but graphically the Asus won out. The margin wasn't big either; we're talking 10-20 fps, max. Even then, that was at the low end of the graphics level 640*480. If I remember correctly.
Really, the best thing would be a good solid board. If you're not going for the uber oc then get a decent board. A Gigabyte, Abit, Asus, something that's already proven itself. A chip in one mb will perform differently than in another.