exhaust? gas mileage improvement? 4wd?

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?

Not too sure on the exhaust thing but with the 4wd, I think it's worth it I use it whenever I'm around tahoe wheter it be in the snow, or the rocky, and dirt roads at some cheaper campsites. I'm not sure about using it on the road though. The 4wd system in the suburban is only part time. This means that the TC doesn't allow for slippage, therefore all 4 wheels spin at the same speed. In AWD systems the wheels can spin at different speeds. Part time 4wd works best in really crummy situations like deaper snow, mud or gravel. Driving on a plowed road, would probrably result in deteriorated handling, and not to mention will screw up your 4wd system really quick. However if you ever get stuck in a ditch or in the mud, you'll miss 4wd if you don't have it.