I also agree with the last two posts. Check out some online reliability info, like MSN's
http://autos.msn.com/home/reliability_ratings.aspx. The recent years of Silverado rate very well.
I just bought mine in April, not my first truck, nor my first used vehicle, but my first Chevy truck, so I talked to some friends who are long-time Chevy owners of many models from the Standard cab short bed V6 to the Crew Cab Long Bed Duramax, all with great things to say. Things I was told to watch out for are pretty much the same things as with any used car or truck - engine or transmission noises that aren't normal, check fluids, look underneath, etc.
Oh yeah, and ditto - go with the 5.3. Happy hunting - I LOVE mine. Check out a Quadrasteer if you find one.
Come back and post what you end up getting!