I would only get it if you can keep the Boxster. The S6 is a very special car but a completely different car than the Boxster. IMHO, the 8 year old S6 looks dated and the thrill of the 435 HP will quickly pass.
If you can get it for a good price and keep the 986.... go for it.
Great Car & Driver write up from 2007;
2007 Audi S6 - First Drive Review - Car Reviews - Car and Driver