Depends on what you want out of your air ride system. You can replace the springs entirely with an air bag/air sleeve system, you can just install helper bags in the springs to adjust the height, or you can do a helper bag system that replaces the bump stops.
The best ride will be from a complete spring swap, but that's the most expensive way to do it, the spring mounted bags are the cheapest, and the bump stop replacement kits will be middle of the road. Have a look here
for some ideas. Also, if you just want a helper bag system check out total load control as well.