Well, here's my take. If you gotta ask, you're probably not ready to install them on your own. It's not that hard but can be very frustrating. Spend money on good gears preferable with the shim thickness etched into the gears. This leaves out all the guess work and makes it much easier.
For 33's I would go with 4.88's. 4.56's are OK but you will be better off with the lower gears.
One tool that wasn't mentioned as a lbs-inch dial type torque wrench. This will be needed to set the bearing preload on a crush sleeve axle. You want some cheater/set-up bearings and a hydraulic press is always nice to have. The D30 requires you to install the shims before pressing on the bearings while the D35 is the opposite. Air tools or a electric impact wrench is good also. (There just isn't that much room to put a breaker bar and an extension when your Jeep is on jack stands. You will also need calipers (digitals are better) or micrometer.
Here's a different take on top speed and mileage. If you're lugging the engine and having to go WOT to get the RPM high then you're using more gas and reducing your overall mileage. This really hurts top-end speeds since you have to rev higher in a lower gear to get the higher speeds. Having the engine work in it's designed powerband is crucial. So, in that sense, you may see better gas mileage and higher top speeds. Really though, you're just regaining what you lost/forgot about.
Make sure when you buy gears you get the gears and a Master Install kit. Also make sure the Master install kit comes with new carrier bearings. Also, if you have crush sleeves it's always a good idea to make sure you have an extra one.
