U can use <A HREF=http://www.pcworld.com/downloads/file_description/0,fid,6241,00.asp>powerstrip</A> Or follow the guide to phisically overclock in the link below.Take care doing it this way
http://www.xbitlabs.com/video/radeon8500-overclocking/

PS:keep in mind that ATI radeon doesn't have any cooling 4 the memory chips