Heres a idea that would work for both teams...
Celtics Get:
Mike James (Expiring Contract)
Brendan Haywood (Expiring Contract)
DeShawn Stevenson (2 yrs)
Andray Blatche (3 yrs)
5th Pick in the 2009 NBA Draft (Harden or Evans)
Wizards Get:
Ray Allen (Expiring Contract)
JR Giddens (Rookie Deal)
Future 1st Round Pick
Here is why it makes sense for the Celts. The Celtics can trade Ray and still get good compensation for him. Almost every player they would get back would serve a purpose for the team, both long term and short term. Both James and Haywood would be good backups for Rondo and Perk and if they somehow dont work out, they are on expiring contracts. Then Stevenson has experience starting for playoff teams and would be ok for a year or 2 until the draft pick is ready to step in. Finally, Blatche would give the Celts a young big to develope and ease the blow of losing Glen Davis.
Now for why this would be good for the Wizards. First, they get a future hall of famer who still shows he has game. He could step right into the starting lineup and give Washington one of the best starting fives in the NBA. He has Finals experience and would take the scoring pressure of Arenas, Butler and Jamison. Plus he himself is on an expiring deal and would give the Wizards a lot more cap room in the future if they choose not to resign him. As for Giddens, he basically is just a throw in who maybe could help down the line.
Possible Lineups:
Bos:
PG: Rondo
SG: Stevenson
SF: Pierce
PF: Garnett
C: Perkins
Bench:
PG: James/ House
SG: Harden or Evans/ T. Allen (If he's not traded)
SF: Walker
PF: Blatche/ Powe/ Scalabrine (If he's not traded)
C: Haywood
Washingon:
PG: Arenas
SG: Allen
SF: Butler
PF: Jamison
C: McGee
Bench:
PG: Crittenton
SG: Young
SF: McGuire
PF: Songalia
C: E. Thomas