I have both , the BE6 in my PC and a BE6-II in the wifes. I had a 550 Katmai on my BE6 but after I flashed my bios to the newest revision it now supports my 700.
One difference in the BE6 has 4 ide plugs. Two are for ATA66 and two are for regular ide. So in essence you can have 4 ata66 HDDs and then have a CD and a burner and two more HDDs for a total of 8.
The BE6-II just had two ide plugs.
The BE6 has FSB steps of 66,75,83,100,105,110,112,115,124,133,150. Might be a few more, but thats most of them.
The BE6-II has FSB steps from 66-150 in one step increments so the FSB speed you choose is limitless for the most part.
I'm almost sure the BE6 is soft menu III and the BE6-II has the soft menu III as well.
There might be a few more minor differences but those are the major players.