Your issue pertains to hardware and the enabling of HD modes in the MS Dash, both prerequisites to setting XBMC's resolution to anything other than SD/ED standard.
And you know what? Rather than posting sad smilies and waiting days on end for an answer, next time try google.com. You wanted to know about "Xbox HD cable" right?
First results for "Xbox HD cabnle" and "Xbox HD pack"... even just "Xbox HD" return the very answer you wanted.
Moving to Newbie Chat...