Rendering as an *uncompressed* AVI doesn’t cause quality loss. But you should know that it can’t be played back on most systems — it’s meant for clips that need to be transferred to another app for further processing.
All HD is widescreen.
What you were working in is called “720p”. It’s one “flavour” of HD. Another is 1080i. AE has presets for those.
In this business you work in the format you’re delivering, the format of your source, or the largest format you expect to deliver in the future (if budget allows). The best format is the one that you’re delivering.
If you’re delivering DV widescreen, work in that format. But beware: your 4:3 footage will have to be enlarged to fit the width of the screen, or you’ll have to show black bars on either side of the screen. If you have to enlarge the footage, it will look softer, but not much worse than DV already is. Make sure you want to do this, otherwise just deliver regular 4:3 DV.
The “big thing” with HD is that properly shot HD looks better than SD (standard def).
If you have HD source, you don’t have to burn HD-DVD or Blu-Ray. You can shrink it down to SD, and burn a regular DVD from it. But know that properly shot HD footage will look better than SD footage, even when shrunk down to SD.