The ASUS 680i boards cannot OC a quad core that well. There are a few people like AMD+nVidia
get 3.5GHz from there quads with the 1403 bios, but they are few and far between:
3.0GHz-3.2GHz will probably be your limit. Its not just the FSB Hole at play here, it is the GTL REF voltage. The ASUS 680i motherboards struggle to OC the quad core. The problem is a hardware issue. The ASUS 680i motherboards do not follow the NVIDIA reference design, while they are great overclockers with the dual cores, they are not in any way good overclockers with the quad cores. This is due to the GTL voltage supplied to the CPU. The rev A1 680i boards released by NVIDIA (rebadged by EVGA, XFX and BFG) fixes this issue. Unfortunately ASUS will not do anything about it. So this is a hardware issue that will never be fixed by bios updates.
Here is some documentation:
Vr-Zone A1 revision
There is a way to modify the board to compensate for this:
Vr-Zone Ketsfield mod
I dont recommend the mod at all. But it does help illustrate the issue.