E8x00 => Penryn, which is built on 45nm process node. E6x00, or even E6x50 are both built on 65nm process node. So while both E6x50 and E8x00 shares the same FSB speed, the difference in process node made them different from each other.
If I remember correctly, current 6x0i chipsets do not support 45nm CPUs, unless its revised. For instance, if your chipset is batch A1, then you're out of luck. Of course, its difficult to determine which version of 680i chipset you have, therefore it might be better to call ASUS support, and ask them about it.