By far the best upgrade you can do to your mountain bike is the suspension, and Push are the best in the business.
They have two main products, the Push Elevensix shock and the ACS3 coil conversion for forks.
Push Elevensix shock
Each shock is hand built specifically to suit the leverage curve of your bike and then tuned to your weight and riding style. It has a dual overhead valve system with two independent circuits that can be adjusted for different trail conditions, each with a high and low speed compression adjustments. You can have one setting for descending and one for climbing, or one for downhill and one for trail. Switch between them on the fly with the flip of the paddle. Each setting is tuned the way you want it to be so there is no set up time, but if you want to play around with the setting go ahead.
Give us a call if you are interested in the best rear shock on the market. We can discuss the options and pricing.