I had an '02, an '03, an '04, and now an '06.
Here's my opinion:
#1 Definitely quieter. I'm not sure what they did, I was reading about details in some review but don't remember what they changed, but it is so much quieter than my previous trucks, and when you shut the doors you can just hear that nice seal. Much more solid. Not to mention the dash just sounds more solid as well, as I think they did a better job putting the '06 together. Maybe less clips or something.
#2 I'm not sure if this is because I went from a 1500 to a 2500, but the ride is much more smoother. My friend asked me when I got it, "Rides pretty rough though eh?" Because it's such a huge truck, but I was like, "Man this thing rides smoother than my other truck. " I think they changed some suspension stuff on the '06 but don't take that for gospel, and it's nothing drastic. I was just reading they changed something but again, that may be the 1500's, so maybe mine is just smoother because I went to 2500, but anyway...
That's about all I've noticed other than the cosmetic differences.