Depends on the video audio bitrate. If you download a low quality sample and convert it to DVD it won't be anything like original DVD-Video quality.
However if you get a high bitrate HD video and convert it to DVD-Video it'll look very good.
If you downloaded an AVI file that was a decent res, and bitrate (1000kbps +) then it should be maintained when you convert it to DVD. Works fine here with Virtual Dub etc.
Liam