Currently the only chipset which supports 2 x 16 lanes is the upcoming Nforce 590, but these have been held up by most manufacturers because the C19 chips have been found not to overclock well enough. The 590 with the C55 chips will apparently solve this problem.
The Intel 975x will only ever do 2 x 8 and the 965 with Crossfire support in the latest Catalyst drivers and a new BIOS will do 1 x 16 in one slot and 1 x 4 in the other.
Current data suggests that there will be little noteable difference between 2 x 8 and 2 x 16.
*WOOPS* I assumed he was talking about C2D. AS those below say, there are plenty of 2 x 16 boards for AMD.
It's not quite fair to say there's NO performance gain, but we're talking circa half a percent here, if I remember correctly. It's not enough to justify it, and an 8x/8x will have more than enough bandwith for you.