We need to trade Ray, have Atlanta buy him out, then have Ray come back to Boston. But alas, decent teams (i.e. teams in the playoffs) don't usually buy out guys.
Some people say we need more size/rebounding/athleticism, but if we trade Ray we then we'll get those but we'll lose shooting, and people will then say we need more shooting. It's like robbing Peter to pay Paul, we'd be fixing one hole while causing another. Pierce and occasional appearance by Delonte West is not enough shooting.
And if we trade Smith for Garnett, we'll have a big hole in the energy/chemistry department. Sure we win the trade on paper, but not on the court. Smith next to Garnett could work very well. Smith without Garnett, we'd be kinda like Atlanta is now, best case 2nd round and out.
Smith/Garnett or Smith/Allen trades alone won't get us to 18. Would need something more.