Ummm yeah you pretty much dont overclock most factory built PCs. I would venture to guess that the BIOS is pretty locked down and you wont have a whole lot of options to mess with. At least when I worked on Dells in the past you certainly could not adjust any FSB or multipliers or anything of that nature.
There's another thread where a dell bumped fsb by way of soldering a few jumpers. Definately not something you'd want to try if you're electronically challenged.