Few things.
Firstly, congrats. No doubt you got a good deal in this market...and the fact you always seem to get a good deal! It's a great car and, in my view, better than a 4200 in numerous ways.
On start up, when cold, the car starts with valves open. Sounds great!
You could change this using the switch Adam has mentioned.
The switch in either guise is definitely worth getting.
There are no secondary cats on a 2006 4.3. The best change you can make is to buy 200 Cell Sport Cats. It honestly sounds epic. Keep the standard rear back box and use the switch to lessen the noise as required. The noise above 4.5k revs is simply outstanding. Below that the low growl is great.
I recommend the RS Fabrications 200 Cell Cats based on the orchestra they produce. Not overly high pitched or raspy as done others.
Hope that helps.