Dwight Howard and Deron Williams are the only all-NBA level talents available right now. The Celtics need to get them both to beat the Heat and Thunder juggernauts that will likely otherwise rule the NBA for the forseeable future.
I think the following scenario works to get Howard and Williams on the team under the salary cap. Please let me know if you see any salary cap flaws that make what follows impossible. (Warning - this hinges on a Rondo/Green for Howard trade.)
Players under contract next year: $30.5M
Paul Pierce: $16.79 million
Rajon Rondo: $11 million
Avery Bradley: $1.63 million
JaJuan Johnson: 1.089 million
Renounce all free agents and options, except for Moore and Jeff Green, to clear remaining cap holds.
Pick up option on Moore at about $800K
Sign Jeff Green to a multi-year deal avg. of $7-8M
Salaries now at around $40M
Trade Rondo + Green (sign & trade) + draft picks #21 and #22 to Orlando for Howard at $19.2M. (Rondo's value coming off these playoffs will probably never be higher.)*
The salaries match so Celtics team salaries stay at $40M
Presuming a 2012-13 cap of about $60M that leaves about $20M space to sign Williams and spare parts.
2012-2013 core:
Williams
Bradley
Pierce
Howard
A starting power forward and bench depth (to go with Moore and JJJ) would still need to be addressed but I think Ainge likes to get his stars in place first and then fill in the blanks later. Hopefully there would be enough wiggle room to bring back Stiemsma.
2013-2014
Exercise option on Pierce (who probably retires), resign Dwight Howard. Use cap space from Pierce option to find a third scorer to go with Williams and Howard.
* In my opinion, my proposed trade with Orlando leaves the Magic in good shape to rebuild with Rondo, Reddick, Jeff Green, and a re-signed Ryan Andersen as a good young core of assets, along with draft picks 19, 20, and 21 (and the potential to trade up in this draft).