Dell, as with most prebuild's, do not allow options in the BIOS for overclocking.
Dell, HP, Gateway, etc. These computers are sold as is, with warranties, and support. The easisest way to keep people from having to use them is limit what you can do with the PC, basically they make is so you can't screw it up. They do not want you opening the case, messing arouind, upgrading and overclocking. If you want a faster computer, they want you to buy one.
Now, there have been Dell Bios hacks, and there is the possibility of overclocking through software.
If you have a Dell, the best place to go for support for your Dell, is the Dell forums.