It appears that no less than TEN VIDEO CARDS were used in that model. http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c01463607
As such even I can't tell you what cable you need. Any shop should be able to inspect that video card and complete this task.
Now I see why the HP support failed you. No one can answer this easy question.