I believe Ray Allen and Kevin Garnett have no intention whatsoever of staying in Boston to play for a fringe playoff team. I think Ainge's decision made up their minds.
So, I'm expecting Garnett and Allen to walk away (ditto for Bass, Pietrus, etc) and Ainge to trade Pierce at some point before the start of the season.
The additions will be the rookies, Green, maybe some player that comes back in the Pierce trade, maybe a young free-agent like Beasley and a couple of mediocre veterans in short-term deals to round the roster.