Burning an AVCHD Final Cut Express project using iDVD

I have a 1080i AVCHD video that's been edited down to 45 minutes in Final Cut Express, I'd like to burn it to DVD using iDVD, but the quality when I export as a quicktime movie and burn the disc is not good at all. I'm not expecting it to be HD quality. I'm hoping for DVD quality, when I burn this it barely looks VHS quality. It just seems like iDVD isn't compressing it well. Would it look better if I exported it to a quicktime movie from FCE and changed the size to a more DVD friendly 720x480? How do I go about doing that?
In iDVD I'm using Professional quality, I have around 20GB free on the hard drive. Should I free up more space before I try again? Any help here would be great.