I almost wrote an essay about this when you asked me, why dont you post it here to allow other people to see?
OR, you can play with your memory dividers yourself and see
EDIT: I found it
Okay in your system, your memory is 1:1 with the FSB (DDR400 is actually 200mhz)
If you make that divider 5:6 your RAM run's at lets say 166mhz when your FSB is 200 mhz.
You raise your FSB to 210 mhz it's now (FSB * 5/6 [0.833_]) which gives you a new memory speed of 175 mhz now.
At 250 it is 208mhz, which is an overclock almost any memory can do and your processor is nwo running much faster.