On 800MHz CPU's the 4:5 is not available. The lowest seems to be 3:5.
I have a E4400 and with BSEL mod it would not boot (I guess on AUTO the dividers are 4:4 and the memory was running at 1066).

Without BSEL the maximum for me is 280MHz FSB and memory at 933 (I have some cheap Kingmax DDR800).

Do you really think that new dividers will be added with future BIOSes? They released F2 and nothing changed.