Glad to see that you are jumping on the bandwagon and getting a second set of tires/rims for the winter. Because you have a 4.2, you might not be able to run 17's because of the size of the brake calipers etc. In which case you might want to just get the tire and swap them out between seasons. …etc…etc..
Again, tirerack.com has a great website for tire options. Personally, I run Blizzaks on my 2001 A6 but, I've heard a lot of good things about Nokian's. The Nokian's are a bit more expensive, but they tend to last longer. For instance, my Blazzaks were $100/tire from the tirerack 2 years ago..and I'll get a 3rd season out of them. Your mileage may vary….
See the C6 A6 forum on audiworld.com for future questions.
Good luck!
PS: I'm jealous of your C6. I test drove a 4.2 when they hit the dealers…what a well packaged car; power, luxury and functionality…all for $55K.