recommendation for quiet split A/C unit
You can probably tell by the username my suggestion, mdshunk brought up a very valid point. the system I would have to reccomend would be the Carrier Infinity. The only problem with going with a split system soley for this purpose is that finding one to fit the application. The best reccomendation I could give you would be to upsize your current system to include you garage if possible and to zone it. The use of the Carrier Infinity system will allow you good comfort, good utility bills and coupled with a zoning system good capabiltiy. As with any installation or retrofit, research the installing company, make sure they size ductwork and equipment properly. If they do a quality installation, you will drive from Callifornia to where I live to shake my hand.
Good luck with your quest.