You have a problem of sorts.
Now is a great time to buy a leftover Boxster and a lousy time to sell your C6 (esp in your area).
Tell me a bit more about your C6 and I can develop some REAL prices and strategies for moving that car.
Regarding the Porsche, the absolute best time to buy this type of car is between 12/26 and 12/31.
Your bargaining power will never be better.
On the C6, the same thing in reverse holds true. Your Porsche dealer does not want that car right now and his offer will reflect that indifference.
Good luck.