|Well excuse me for misreading your post.|
Maybe you should try working on your search skills because I found the answer in less than a minute.
The XPS 420 is NOToverclockable via the BIOS!
Here's a clip from a review:
"Curiously, unlike other XPS systems there are no options in the BIOS for overclocking -- possibly due to the refocused target group, but still a sad omission nonetheless"
And by "target group", Dell means the "Creative Class" -- "artists, designers, film-makers, producers, architects, builders, engineers, scientists, professors".