Look at the VGA library thread (in my sig). NTSC is supported, but the bandwidth for NTSC is actually nearer 6MHz, so needs a sample frequency over 12MHz to generate (I use 14MHz). And as the VGA colour mode is half-resolution I use 14MHz for that too. So the bandwidth is the same, but as NTSC is a more complex signal it actually takes much more out of the Due, not less. (VGA can be done with DMA alone, but NTSC requires processing as well to do the mathematics).