First you need the HD video cable, it has component connectors, or a 3rd party component cable.
Then you need to go into the dashboard and enable the HD resolutions under the screen settings which will be available once you have a component cable connected. I think you only get the option to do this on NTSC xboxes but not sure on that point.
Edit: oh, I see the bit about "if you can't access the original dashboard" well, some dashboards like new versions of XBMC allow you to do it too.
