Scott,

If you plan to make this a "man cave" rather than a garage, complete with carpet and TV, what I would do is run an extra A/C duct from the central unit to the garage. You said yourself you won't have the garage door open much (except maybe those 5 days of spring we get in the south before summer kicks in). You can even install a one-way air flow gate into the duct to keep garage air from creeping it's way back to the rest of the house while the system isn't running. It would probably be cheaper than buying a window unit, and if your garage is like mine without windows, you wouldn't have to cut a hole in the wall to install it. Due to restictions on window units in my subdivision this is the route I plan to go.

Matt