Burning Compression, loss of Quality



I have a question regarding DVD Burning quality. I am using Roxio EMC 10. They have an option to burn to an 8.5 GB Disk size, whereby you can set the quality to HQ. Then I use Video Copy & Convert to compress the file down to fit on the disk and then burn it. Recently having problems with the burns not working. I have also experimented with using Instant Copy and burning PDI Files from Folder format (VOB, BUF, IFO Files). Basically I am looking to get an AVI File onto a disk with losing as little quality as possible. How about burning PDI Files vs ISO Files vs Folder Format(VTS_ files). Should I be trying to make my PDI files or Iso image stay under 4.7GB, therefore no compression will be needed. Hope this post isn’t to confusing? Kind of a novice at all of this and trying to learn. Any advice, suggestions, tips would be appreciated. Thanks