First of all, you’re taking almost a 3X quality hit but going from 1280 x 720 down to 720 x 480 🙁
Then you’re converting your pristine footage to MPEG-2 which is another quality hit.
The only way to maximize quality is to do all your renders at a CBR of 8,000,000. Some folks say that you can get away with 8,500,000 but I prefer the lower number.
At that bit rate (8M), you can only get 1 hr. 11 min. on a DVD (using Marks calculator with the safety margin set to 5%).