I have a raised ranch built in 1978 with radiant heat, hot water base board heaters and a whole house fan. I'm looking to upgrade my house w/ Central Air. Is this even possible w/o ripping the house up? How/What/Where do I start? My friend told me I need a new furnace to go along w/ the central air, but I'm content w/ the heating system I currently have. Is this really necessary?

Any advice, tips, or information is appreciated.