While I was deciding on a new set of tires for the A6 I saw a pic of and A6 with 18" wheels. Damn it looked sharp, and now I thinking it's time to move up to 18"ers. If you have any advice to share (do's, dont's, size recommendations etc) I would appreciate it.

Audi offers the car with 18's as a factory option so won't be a problem fitting those at all. 19's should fit too with stock ride height but the bigger you go, the more you have to worry about bending if you live in an area with bad roads and minor effects on handling, turning radius etc.

I had 18s on mine, Worked fine and the ride quality was surprisingly nice. I went from the 16s to 18s and it was a huge (positive) difference.

18x8s should work. Depending on the offset of the new rims, you may need spacers. 35mm offset should work fine. I had 43mm offset wheels, and had to get some spacers.