You probably don't need to reformat, but I always do when I upgrade (new MB & CPU ?) like that.
Memory Dividers I believe are under "CPU Configuration" then "DRAM Configuration". You'll need to change to "Manual" to access them, then you'll have options from DDR200 thru DDR600 - under "Max Memclock".
Commonly used:
DDR400 = 1:1
DDR333 = 5:6
DDR266 = 2:3
DDR200 = 1:2
I believe those are the correct ratios, it's been I while since I've looked into them.