I totally missed that part about Mobility Radeon XD
Anyway, its very unadvisable to overclock a laptop, even if it supports it. Overclocking puts a strain on all the parts in the computer (or laptop), and in a laptop, everything's already strained. Heat management is a pretty big issue in laptops, and as you can guess, isn't very well managed to start with in laptops IMO (I know some laptops still crash due to heat when put on load... so...)