HVAC question: I live in a ranch house with central air. The air was
added after the house was built and runs through the heat vents. My
question is would I get more efficiency and better cooling results if
I had ducts in the ceiling? I'm thinking something like commercial
flexible/insulated ducting to the attic and then down to a vent in
each room. There would of course need to be a way to toggle this
on/off for heating season. Is this practical? Is it worth it? What
would it cost approx (three bedrooms, 1,500 sq feet)

May I suggest posting your question on this Web site

http://hvac-talk.com/vbb/

along with some details, such as state / county where the property is
located and some more tech specs of the system installed
(manufacturer, type)?

I am sure what type of heat vents you have in your house. However you
can convert existing home (without cooling system) to central cooling
system through air handling unit in attic and high velocity flexible
ducting system in the wall and ceiling. There is a product "Unico
system" in the market that is very popular in this area. Flexible
ductwork are smaller in diameter and can be fish through the  stud
walls. There web page is www.unicosystem.com
You could also find there other informations such as local dealer,
installer and cost etc.