I think you would have to get a very new, very effcient, and probably very expensive AC unit in order for it to be cost effective, energy wise. Your house needs to be well insulated, too, that's just as important. So my gut instinct is to stick with fans, because that equipment is so much cheaper than a new AC unit.
As an aside, my parents live in FL where AC is not optional. When they moved into their house, energy was running about $400 per month. After a new central AC unit, it went to $300. After $40,000 of new wall and ceiling insulation, new windows and doors, new siding and new roof, their energy costs run about $75 per month.
Trying to turn hearts and minds toward universal healthcare, one post at a time.