Hi magnus,
First, try 528x480 (kvcdx3), do file size prediction, and playback your sample to preview the quality.
If you're happy with the quality: "Do not pass go, do not collect $200.00"

and encode your movie. Case closed.
If quality is so so, then lower your resolution to 352x480, and run prediction again.
Most probably, you'll have excelent quality 90% of the times at 528x480, and fallback to 352x480 if it's a very active (or long) movie, where 528x480 just takes up too much bitrate for a descent quality.
-kwag