DVI on the Radeon to HDMI on the HDTV, via an adapter or adapter cable.
2nd best: Use a DVI to component video adapter (available from ATI for
some models of video cards only), then component video to the HDTV set.
It's unlikely that you will be able to do 1080i via an analog VGA port,
although it might be possible on some models.
Ultimately, this answer is to a degree dependent on both the model of
video card and the model of HDTV that you are using.