Graeme Nattress posted recently that he thought the "in-camera" down-convert was actually NOT the best quality.
i know there are plenty of people who would have used the in-camera down-convert (including me), if that helps.
if you're using Mpeg Streamclip because you still have FCP4 or something,
there is a "Better Downscaling" button
btw, DONT use the Export to DV as that's really "Export to DV STREAM".
that's the iMovie flavour of DV, and you don't want it.
sound is muxed, and cant play in FCP.
use Export to Quicktime, set to Apple DV.
DV is one & zeros, so isn't affected by any quality sliders, so you can ignore that if you want.
good luck,
nick