It's exactly as Kemp described earlier. Current video card technology doesn't even come close using all of the available bandwidth on an 8x PCI-E lane, and 16x is just complete overkill. I would say that not even a pair of 7950GX2's could utilize all of the bandwidth from dual 8x lanes.
And to be honest the move from AGP to PCI-E for video cards was and almost still is something not even necessary. AGP is a great design with a lot of bandwidth. The lanes are a bit slower than PCI-E but this doesn't make much of a performance hit (66mhz IIRC compared to PCI-E @ 100mhz). I remember a while back seeing some tests based on cards built for both AGP and PCI-E and the AGP cards perfomed just as well as the PCI-E cards. But as with anything in the computer world, it's all about future proofing. Sure, 16x PCI-E slots might be needed in the future but I'm almost positive you will have upgraded your mobo more than once before you actually NEED dual 16x PCI-E.
And a quick search I wasn't able to come up with anything for the Asus board either, sorry mate.
Edit: Still a bit off topic, sorry. But I was able to dig up a pretty good article on AGP vs. PCI-E
HERE. As you can see, AGP holds right in there with PCI-E until bus speed becomes a factor at high resolutions trying to run max antialiasing. It would be an interesting shootout with a pair of newer generation video cards built for both PCI-E and AGP. Maybe we could set something up?