If you are runing a 1:1 ratio for your ram, or do not have your ram locked. Your ram will run at fsb x 2. Since its DDR2.
Ie my fsb is 425. My ram ratio is 1:1 so my ram is running at 850mhz.
If I lower my fsb to 400. My ram would be running at 800mhz.
You might wanna check out http://www.pcstats.com/articleview.cfm?articleID=1804
to start with.