If you can read German, there is a pretty good writeup in the FAQ. If not, here is one in english:
http://www.audifans.com/twiki/bin/view/Audi/BuyersGuide

Cheers,

- Jeremy

1990 V8Q Zyclam Portland, Oregon (USA) http://www.MyV8Q.com/ Fun, Fun, Fun auf der Autobahn