There is no such resolution in iGO

Apart from the fact that you should take into account the standard resolution ratios (scaling), iGO can only display pre-defined ones

800x480 exists, which means that you have not set the sys.txt file correctly
Change values to bolded:
highres=0 (leave it or try with 1)
screen_x=480 > 800
screen_y=272 > 480