I drive a 1999 A6 2.8L Quatro.  I'd like to lower it for a sportier look.  I have looked into Eibach and H&R springs. Are there any other brands that I should look at? What is the optimum lowering distance so that it looks good but I won't have to worry about it bottoming out all the time?  And, if I replace the springs will I have to replace the suspension to because it is so soft?

The Eibachs are the best choice.  They're springs and dampers (shocks) are specifically built for each specific model of A6 and many have raved about their quality. 

You can go to Eibach's web site and get the exact part numbers for your car.  They make springs which will lower about 1.2" as well as custom dampers designed to work perfectly with the lower springs.  The "Pro System" includes both.  Most people I've spoken with suggest replacing dampers at the same time as springs, since there's no additional labor involved and the demands of the new springs will usually wear out the OEM dampers rather quickly.

http://www.eibach.com/index1.htm