Text mode, Virtual PC 2004 and full screen mode

Every now and then someone notices that if they have a virtual machine running in ‘text mode’ (like DOS) and they put it into full screen mode – they will get a small black band at the top and bottom of the screen.  We display ‘text mode’ at 640 x 400 resolution (it is natively 320 x 200 – however we double the display to make it more readable when in windowed mode).  The problem is that a number of video cards do not support running at 640 x 400 – but report that they do support running at 640 x 400.  When we try and use 640 x 400 mode on these video cards bad things happen (from corrupt displays to crashing drivers).  To try and avoid this, what we actually do is put the video card into 640 x 480 and display 40 black pixels at the top and bottom of the screen.