The 4000 was around 60% faster than the 3000. And the 5000 is around 40% faster than the 4000. So with the 5000 you should be getting around 224% the performance you got on the 3000.Sorry your right! This probably means that there would be a somewhat noticeable performance increase?
http://www.notebookcheck.net/Mobile-Graphics-Cards-Benchmark-List.844.0.htmlNotebookcheck rates the HD 5000 above the GT 540M which was used in the 3rd version of the Alienware M11x. I think the new Airs should pack plenty of punch when needed!