Since I'm still trying to finish mine, I'll chime in on this....
1ownerT has it right with the thicknesses of your walls. 2x4 construction means a 3 1/2 inch batt. Nothing thicker. My garage is getting R-11 on the walls and the ceiling, vapor barrier (.2 mil.), and then 3/8" OSB over that. Pictures are in my project gallery. Somewhere in there.
Couple of things to really remember have already been listed, but I'll re-state them. First, you must adequately vent the attic to rid that area of excess heat and moisture. I know theres guides on how much venting you need, but if you go to a local Home Depot, Lowes, or Menards, they can tell you how much venting you need. Just have the full garage dimensions when you go.
Secondly, before you button the walls up, best thing to do is run all the wiring you think you may need. It's a real pain to pull it back apart to run another circuit after the wall is finished. Trust me on this. I've been there, done that.
Lastly, you did not mention if your garage is detatched or not. If it is attatched, you may need to have drywall put up between the house and the garage to meet your local building codes. Most areas that I've heard have that as a requirement in new construction.
In a while, Chet.