The spacer is installed inbetween the upper and lower intake manifolds.
It is used to increase the airflow of the intake by giving more clearance around the injectors, etc.
See the shiny bits under my TB?
There is some debate about it's usefullness. The guys who are running rebuilt or 385's seem to put them on. The guys who are running ALL stock, don't seem to think they work. The guys here who have em say they add to the butt-o-meter results.
Personally, I probably wouldn't do the upgrade all by itself. I did mine because I was already in there upgrading the spider, so WTH why not. I subscribe to more air is better. So if you are upgrading anything else especially anything air related like CAI, bigger TB, or exhaust headers, etc, then the spacer would be a good thing to add, otherwise meh.
What else do you need?
I can think of a few things you don't need, but may want....
Bigger TB
CAI
E-fans
UnderDrive pulleys
As for PCM4less? IDK, I really enjoy my BBT for my Yukon, but now I'm over at trailvoy.com for my wife and they really like PCM4Less.