When KG's contract runs out, he'll be done.
With Ray Allen, the decision will depend on a number of scenarios. His expiring contract could be a nice piece... especially if an Amare Stoudamire, Chris Bosh, etc talent becomes available from a desperate team.
Realistically, I think Allen is resigned for a fraction of his salary, say $16/2 years. And then he walks into the sunset around the same time as KG and Pierce.
My main thought looking into the future is that the Celtics have only 1 core player to build around (Rondo), everyone else is a role player and the C's need to make sure they don't overpay them. For example, when Perkins contract runs out, I'd try to resign him for 7 million a year (35/5). If he (or his agent) think they will get $50/5, they are nuts, and we'll find another center.