Dell as well as most big box companies generally give no ability to overclock. You have to have settings in the BIOS to set the FSB/BCLK and voltage.
What EXACTLY are you trying and with what program or where are you trying to make the changes?
i have been trying to use setfsb i just hope to up the cpu from 3.4ghz to something like 3.6 i dont really know much about it its just something i want to know and try out as a hobby i managed to overclock my gpu from 700mhz to 800mhz and is not going above 73oC i know just hope i can overclock the cpu