If you're exporting for DVD, don't use Quicktime Conversion. Export to Quicktime Movie, using "Current Settings" (which will match your sequence settings). Bring the resulting file into iDVD and it will handle the rest.
Alternatively, if you're using Final Cut Pro*(as opposed to Express), you should encode the DVD video and audio streams with Compressor, as you will typically get better results. There are presets for different quality levels built in or your can customize your own. Then, you can bring those files into iDVD or DVDSP (for Pro) to author the DVD.