Well, you won't really need the Radio Shack Sound Pressure Level Meter (either the analog or digital meter will do the job) until you add the 500, surrounds, and center channel. It's used to balance the volume levels of all those speakers, so that, when seated in your favorite spot, all those speakers are equally loud. Once you add the 500, you'll need the meter, and here are some calibration tutorials.

Essential Accessories: The Radio Shack Sound Pressure Level Meter

A Quick Overview of Home Theater Calibration

Calibrating Your Home Theater System

How To Set Up a Subwoofer (or Subwoofers) for Home Theater

Subwoofer Setup Guide by Bossobass


Jack

"People generally quarrel because they cannot argue." - G. K. Chesterton