Dell locks the bios usually, and Overclocking that cpu is pointless, you`ll just most likely damage it if you don`t know what you`re doing and there isn't a lot of room for ocing it in the first place. You`d need a 3770k and a new board.
New MOBO new CPU and new Windows licence because Dell OEM is notgoing to work - basically new build. When people buy Dell they want simplicity and no headache. Unfortunately I am having trouble finding info about possible overclock ways for Dell locked BIOS, I am not saying it is impossible, but not having this XPS myself it is difficult, so far the only promising approach is through PLL numbers. If this fail, then 95% is NO answer, FSB is possible but very limited, say from 3.4 to 3.6 and that is it. AMD is much easier, even in locked Dell !