The first winter after I finished my new garage, I had some unplanned automotive work to do. It was cold, so I plugged in the two 1500watt heaters I have to warm up the space a bit before getting down to work.
I quickly found that I can only run one heater at a time...and some power tools if run together with it could also make it pop...all 6 outlets were installed together on one 15A circuit.
IMO, the right way to do it would have been to install no more than two outlets per circuits and to make them all 20A on 12 gage wire. I'm going to be retrofitting mine eventually...my first priority is better lighting.
The other thing to consider while you're doing this is your lighting. I quickly realized that a few 100W lightbulbs are fine for walking around, but grossly insufficient for working in the garage. I'm interrupting the lighting circuit and adding two outlet boxes to the ceiling, which I'm going to hang two cheap ($20) 4ft flourescent light fixtures off of. I'm using one fixture per outlet, which leaves room to easily double the overhead flourescant fixtures if but two of them aren't enough light. YMMV