It occurs to me that one thing that Celts could do with (some) of their cap space this summer, if they indeed trade Rondo for Gasol, is take on Marion and Odom from Dallas for nothing. Marion and Odom are old, but they're both capable of being solid contributors; Odom in particular I think is underachieving this season because he got off on the wrong foot and wasn't in shape due to the lockout.
Get Odom back playing next to Gasol and give him a full training camp to come together with the team, and I think he could slot in well as the starter, or as the 6th man behind KG (returning in a limited role). Whatever works.
Marion would be a very solid backup to Pierce. If we signed Jeff Green and re-signed Pietrus, we could have a large and multi-talented wing rotation with Green, Pietrus, Ray, Pierce, and Marion (with Bradley getting some minutes at SG, too).
If we trade for Gasol, that means we're committing to winning now, and moreover, committing to being a half-court team. Marion and Odom would make a lot of sense. Sign Andre Miller (or perhaps Nash, Billups, Felton, or Hinrich) to a cheap deal to run the show, and we could have a nice, if very "veteran", team on our hands.
Other players we could conceivably sign to reasonable deals to play in that PF spot, either ahead of or behind Garnett: Humphries or Jamison. We could also just re-sign Bass (once he opts out), and sign a better backup center (though the options there are much more limited).