With the 5th pick they should draft LaMarcus Aldridge, trade Harrington for a pick from a top 12 team in the draft, with that pick draft Marcus Williams (if available) or Rajon Rondo, and sign Joel Przybilla in the off season. With these simple moves the Hawks line-up will look like this:
Marcus Williams/ Rajon Rondo-Tyronn Lue
Joe Johnson-Salim Stoudamire
Josh Smith-Josh Childress
LaMarcus Aldridge-Marvin Williams
Zaza Pachulia-Joel Przybilla
This would be a very young and deep team that will still be under the cap. They should probably sign a veteran PF or SG as well. With the cap space and all the young talent, the Hawks can then make a move for Chris Bosh later.