Think of the air flowing in the duct a lot like water running thru a hose. Attach a 1/2" hose at the end of a 3/4" hose- what happens? You still have a water feed psi of 65, but where you attach the smaller hose the velocity increases, but with an increase of velocity you have a rise in psi at the source (the 3/4" hose) and any psi rise means less total gallons per minute.
The same happens in the ducts. The velocity increases, but the total cfm goes down and the static psi rise is at the air handler. This drop in cfm effects the whole systems capacity and it's ability to deliver the efficiency and will cause all kinds of problems such as icing up and compressor failure.
Upgrade the attic insulation, have the walls checked for insulation and upgrade that as well if you can and since windows are the next biggest problem with cooling and heating a home than the attic- see if those can be replaced or tightened up. Then have a manj load done (this is critical).
If the home is made more efficient, you may be able to save money by keeping the old ducting and the older system (which isn't very old at all to begin with) instead of trying to buy a bigger system which will eat more $$ cooling a loose house.
Compare that to an ice chest. What is a better deal- buying an uninsulated ice chest for $10.00, then having to buy $30.00 worth of ice to keep things cool all day, or buying a $20.00 insulated one and spending $1.50 to keep things cool?
I never like systems in an attic that isn't large enough to stand up in because those attics are hot and the system cannot be serviced as it should. However yours is already in the attic and if another will fit and repairs can be done, it's a lot less costly than re-ducting. A closet in the area is a better choice than outside.
Package systems are nice but the duct is insightful and again not the desired method if at all possible.

daddo,
Thanks for the explanation regarding the ducts, CFM, and tonnage... I appreciate the way you did it in laymen's terms (I need that). I have recently replaced all windows with impact resistant 3/8-1/2 inch thich glass EnergyStar rated...

What about adding return registers to the rooms on the side of the house that does not cool properly? How many returns should a 2,000 square foot Florida house have (and where)? I know that each situation is different (and realize the load calc is needed). The house has only a total of 8 cool air supply registers (plus the one return).

I believe there is no filter in the air handler (only at the return). Is that normal? Is that OK? It sure makes it easier to clean (instead of venturing into the 130 degree attic). I am realizing that all of these issues will hinge on getting an honest, knowledgable contractor to evaluate my complete AC picture. Thanks for your past and future resposes!