when you increase your fsb speed your ram speed increases as well.
if you have your ram set to its stock speed ie. DDR400 = 200MHz, then when you increase your fsb your ram may only allow it to be increased a little before the ram speed limits any further increase.
in bios you can set your ram to run slower than stock speed ie 166, 133 etc. which will give you headroom to increase your fsb more.
eg. at stock ram speed I can only increase my fsb to 238MHz before unstable.....but seeing as though I set my DDR400 RAM as 133MHz then I am able to run my fsb at 300MHz.
Seeing as though increase in fsb = increase in ram speed, starting the ram speed lower means you can increase the fsb without having your ram limit your system too quickly.
seeing as though I set my RAM at 133....when I increased my fsb to 300, my ram increased to 196 which is just under stock speed anyway.
this table will give you an idea of what speed your RAM will run when your fsb is at certain speeds with RAM dividers.
Edit: to display your system specs go to "user cp" at the top of the page and select "edit system specs" and enter your stuff.