You can use the memory tab of CPU-Z to check your FSB: DRAM ratio, eg 1:1.
In the bios the information will be presented slightly differently, and might be referred to as the system memory multiplier, ram divider, or words to that effect, but varies according to mobo manufacturer. On my Gigabyte board you need to set it to 2.0 to get a 1:1 ratio, if left on auto it defaults to 1:2 with 200 fsb processors and DDR 800, and quickly takes the memory to unworkable speeds as you raise fsb.
If you use CPU-Z to find your current ratio, it will help you to make sense of the options in the bios - a 1:1 ratio could be described in the bios as 1:1, 2.0, sync or something else entirely, depending on manufacturer. If in doubt read a faq for your mobo.
Its best to leave it in sync until after you've found a stable fsb for your processor. Then if necessary you can raise the divider/ratio to bring your memory back up to its rated speed.
Hope that makes it (a bit) clearer. Good luck.