What Windows you have.
This could be very interesting thread - I can help to overclock Black Edition in ANY Dell model, XPS might have partially unlocked BIOS, or might not.
I have a wild guess to take your CPU from 3GHz to about 4GHz, how about this?
Also, as stated above we need much more info, Video Card (GPU), memory (RAM) size and speed, purpose of your upgrade, your budget and your geographical location, I mean country, because getting parts can be pain if you live outside of USA and we don't know this (happened many times before).
IF you are willing to go this way, please get some image hosting like imageshack account just in case, it is free, if you will need need to post pictures, just in case.
Start from getting CPUZ...