I would hold off on the box, and replace the rest of the front end at the same time. Without replacing the worn tie-rods, centerlink & idler arm, you're only going to gain half of what you could. Do all that together though, and it will literally feel like a brand new car. That's just my .02 though. Not sure if that is even an option for you right now.
I would say you cant go wrong either way. Both the arms, and the box will make a dramatic difference in how the car feels.