Since it is a Dell, they have a locked down BIOS. The only way to overclock would be through software, such as ClockGen or SetFSB. If the PLL isn't in those programs, you are pretty much out of luck. There may be some pad-mods for these, but it probably isn't worth it.
Something to consider if your PLL is in those programs: The board may not have locked PCI/PCIe buses. So, as you raise the northbridge speed, you are overclocking buses that shouldn't be touched. This can create massive instability and hard drive data corruption.
Honestly, it isn't worth overclocking OEM systems, at all.