Just about anything with ATI's Xpress200 with crossfire support will do that. I have a Asus A8R-MVP that shares the x16 into 2 slots when you go crossfire, or presumably just have 2 x16 cards. In my case, the second slot is fitted with a terminator because I believe that you don't actually get 2 x8s, but a shared x16, there's a difference.
Something with ATI's Xpress3200 crossfire chipset like the Asus A8R32-MVP, has 2 dedicated x16 lanes so whether you use one or both slots, you get the full x16 lane for each card.
A good number of motherboards restrict 16Xslots to video cards only, so an 8x card will fit but it wont work due to bios limitation. The 680 i would work, and if you wont be using crossfire should be fine.