Help me choose AC model
Carrier is one of the better brands.  (However, most brands are pretty good these days.)

If you plan to stay in the house for years, look at systems with a higher SEER rating, as those are more efficient.  A more efficient system is generally more expensive, but it can pay for itself with lower power bills.  

Units with a >16 SEER rating are eligable for a 2011 Tax Credit.  My local power company also had a rebate for high-efficiency HVAC, so look into that too.  (There may also be state and local tax credits).

Make sure the outdoor and indoor units are a matched/supported configuration, with an AHRI certificate.  You'll need that for any tax credits and rebates.

Some features to look for:


Higher end systems often have a better warranty (5 year vs 10 year).
On the outdoor unit, a two-stage compressor is nice because on moderate days the system doesn't run full blast, which is more efficient.
On the outdoor unit, a scroll compressor is quieter.  This can be nice if the outtdoor unit is next to your deck or window.  (Some units can be pretty loud.)
On the indoor unit, variable speed fans are quieter, more comfortable (they gradually speed up and slow down vs a sudden on/off), and help regulate humidity better.
A higher end indoor unit often supports a thicker air filter (e.g. MERV10 or MERV16).  Your typical 1" air filter is just some mesh over a plastic frame.  A filter like that can trap clumps of dust and hair, but not fine particles.  A higher end air filter is a few inches thick and looks like the air filter in your car.  They can trap finer particles and improve the air quality in the house.
A higher end thermostat may be necessary to fully take advantage of the features provided by your indoor/outdoor unit.  Higher end thermostats also provide fancy features like extra dehumidification, touch screens, scheduling, etc.


However, all of these features can be irrelvant if the installation was poorly done.  A top-end system can have tons of problems because it wasn't installed properly.  (e.g. The dehumidification feature can be a moot point when you find out the installer took shortcuts while wiring up the thermostat and air handler.)  Find a reputable HVAC contactor that is experienced with the brand/model of system to be installed.

Ultimately, you need the following:


A system that is sized properly for your house (tonnage/BTUs).  If the unit is oversized or undersized, you will have problems.
A manufacturer-supported combination of indoor unit, outdoor unit, and thermostat.
Properly sized ducting (both supply and return) that is insulated and doesn't choke airflow
The installation to be performed properly, in accordance with the manufacturer's reccomendations.