Mike;
I just went through what you are going through in April. My home is in
Va. I replaced my unit with an 18seer york heatpump. The most important
thing is to get a Manual J AND a Manual D done. The Manual J is
going to check for heat /cooling loss through your windows and walls.
The Manual D will check your current ducts out.

Normally, a new higher seer 3 ton unit will put out better more
efficient cooling than a 20 year old unit. It is very IMPORTANT not to
oversize your unit. If you oversize your unit, it will run half as much
and it will cool the house but do nothing for the humidity. All the
cycling on and off is harder on the motors and compressors. Your indoor
air quality will also suffer.

The newer models now have variable spped fans and 2 speed compressors.
The units will run continously but at a lower speed. This will provide
for better humidity control, and better indoor air quality because the
air is constantly being filtered. As a result, it is much better for
indoor air poluttants and allergies. With some of the higher seer
units, you need larger ducts to handle the airflow. A manual D will
insure your current ducts are ok.

For my 18 Seer York unit, I paid 11000 installed with a 10 year waranty
on everything. The new units are not cheap. Part of it is Katrina, and
the other part is that you are going to save so much on utilities, that
they are not gonna give the units away. Here is a pretty good site that
can tell you how much you can expect to save with a new unit.
http://www.hvacopcost.com/

Also reccommend that you post at www.hvac-talk.com in the residential
section. Their are a lot of techs over there that can guide you towards
making a good decision.

Take Care
Pat