I have YFZ rear and Banshee fronts and, I noticed a world of difference. especially with the YFZ rear. If you are looking to do a full shock upgrade for under $150.00 that would be the way to go. If you want to spend more, and get either a diamond j kit or wider a arms go with YFZs all around. But that is going to cost you around $400 to $700. If you want some really nice shocks go with ekla or works but that is going to cost you upwards of $1000.00.
Just depends on how much money you want to spend.