If you are changing out the springs you might as well do the struts and shocks as well. The struts must be taken out and disassembled to get new springs in there and the rear shocks are just two bolts to take out. Really very very easy.
I would really recommend not replacing just the springs. Your stock dampers are 4 years old and are probably worn out a bit. Putting shorter springs on there will make them wear out much quicker as they cannot take advantage of their full range of movement.
Save your money and do it right. Do it once. You will get much better handling as well. Good dampers matched to the springs help with the transition of weight to the wheels of the vehicle. Too soft of dampers and you will create a bouncy ride in the straight line and a wallowing ride into corners.
Look into the Eibach pro-kit or the SVT kit. The Eibach drops you 1.5 and the SVT .75. Both will yeild awesome improvements in both handling and look over your stock setup. And will last a long time.
ah didn't catch that. Massive sells a kit that allows you to use 05 and minus suspension kits. The difference was with the strut design. You could check out Hilbish Ford though. Mike sells the ford racing performance parts kit which is comparable to the Eibach pro kit. Also a 1.5" drop with very similar handling characteristics. He has kits that will fit your 06 just fine.