Your in luck. This is a great message board and you should receive lots of comments. Also, try the search button to research earlier discussions on this same topic. I'm a fairly new member and found the information on this site invaluable for making decisions on modifications to make to my truck.
If you are budget minded, try calling your local Hummer dealership and see if they will sell you some stock BFGs off of an H2. The H2 comes with BFG AT KOs at 315/70/17. Many buyers want the larger 20" or 22" rims with low profile tires. So, the dealership replaces the rims and tires and ends up with many of these stock H2 wheels sitting in their parts department. You may get an awesome price on a set of these and this may help you make your decision. I've heard of prices as low as $50 to $80 a tire. With the money you save on tires you can purchase your body lift and a microtuner.
I just put a 3" Performance Accessories Body Lift on my 2003 Fx4 SCrew with a set of these tires on my stock rims. I also added the 1715 Microtuner to recalibrate the speedometer and the transmission shift points. I've only run one tank of gas through the truck so far. So, I don't know much about the effect on mileage but so far so good. I can tell you that the truck drives much larger but the ride is pretty much the same. I did not crank the torsion bars so that may account for the ride staying fairly smooth. See the post 315/70/17s Fit? Mileage? for more information.