DELL laptops are generally not overlockable except the recent XPS M1710's.
In terms of GPU overclocks, I have a XPS M1210 with the 7400 GO and I tried overclocking my VGU and it made my system very unstable. I wouldn't advise overclocking anything on a DELL laptop just because everything is hot as **** to begin with, and the specs are allready designed to give max performance without creating an unstable environment.