exhaust? gas mileage improvement? 4wd?

[blockburna]

I was considereing getting a cat back exhaust system for a 02 Suburban 5.3 (I will be buying soon) and I wanted some opinions on different systems. I do not want anything thats to loud on the inside because I will be traveling with it.
How much would a cat back or intake help with gas mileage? What else helps with gas mileage?
Also do you know of any suburban / tahoe / silverado / avalanche web sites?
Do you think a 4wd is really worth it if the truck will only be driven on road? I was thinking I would be better for snowy or rainy weather. How many of you actually use 4wd on road?