normally with the big brand OEM's the mobo's have little or no ability to overclock.
i've heard that sometimes dell can be an exception, simply rebadging boards from other companies and leaving overclocking options intact.
if you've tried to increase the fsb with no luck you might have a "no overclock" board.
how did you try? in the bios?
check the board itself for jumpers. there may be an "auto fsb" jumper which is preventing the changes in the bios.