Higher FSB is not always better. There are timings to consider. Even if the FSB is going faster if it has to wait too many cycles then you negate the speed benefit, and have to run higher NB voltage.
Have a read.
http://www.anandtech.com/mb/showdoc.aspx?i=3208&p=9
tRD is the timing that has a huge bearing on how effective your FSB speed is. Running 400 or 450 w/ a tight tRD may give you better performance than faster FSB speeds.
Try several settings and use Everest benchmark to make memory bandwidth comparisons. Try to get your RAM as tight as possible at each setting to ensure you can make accurate comparisons.