FSB controls RAM, CPU and a couple other things.. When you overclock it overclocks EVERYTHING.. If i had ddr400 (pc3200) ran it at 200mhz (like its supposed to) then overclocked to 263mhz (63mhz overclock) my ram would run at 263mhz. This would blow up my ram instantly.
So, you downclock your base memory so when you overclock it runs it at normal speed, or just slightly over. I'm not sure what about this is so hard to grasp
it used to be back in the day with p3 that when you overclocked FSB it even overclocked your hard drives...
edit: oh... and i didnt say what the divider was because my motherboard doesnt have a divider like some of the others.. I Know the asus a8v, a8n-sli, gigabyte k8ns*, and a couple others do it like that... but not mine.. i have to set my mem clock manually.