Hi dudes, firstly your TV has to be 480p or HDTV compatible,and you have to use the advanced AV leads and component or HD input into your TV,into the progressive or HD compatible input.
Then your xbox needs to be in NTSC mode using the enigmah switching app,then you need to set the 480p/720p/1080i settings in the original xbox dash i.e xboxdash.xbe.This can be tricky if the xboxdash doesnt output in progressive mode,but you can blindly navigate the menu, like this : down 2 press a(settings),down 3 press a (video),down 1 press a (480p) ,press left then a (this screen asks if your tv supports 480p),then power off and power on,avalaunch should now tell you have 480p enabled.
There are further settings for 720p and 1080i but very few games that support it,I have found 480p works best all round,I have just hex-edited the xboxdash.xbe to work in 480p using the forums here.
Hope that helps.