OK, with a Dell you can't go into the BIOS and the change the FSB or Vcore settings, (or memory divider, or vdimm) so that means no overclocking options. It's the same thing with HP, Compaq, IBM, Gateway, and Sony. No overclocking on these machines.
The only way to overclock a dell would be via software but i wouldn't really go that route, i dont know of any software i would trust to overclock a PC. Maybe someone else here knows but i don't.
Only way to overclock would be to do it on the computer your building yourself.